The Walls, Hadrian's and Antonine
Both walls were built by the Roman occupying  legions and had a military purpose, to keep those "Caledonii" north of the wall from overrunning what was south of the wall.  The Antonine wall was intended to push the line of protection further north, but as legions were needed for service off the Island the Romans retrenched and soon moved back to the earlier line.  By 410 AD all of the occupying legions had been sent to fight on the mainland and the defense of the Hadrian line was also over.  The path southward was open for the Caledoniis (sometimes also called "Picts" by the Romans because the male Pict fighters painted and tattooed (PICTured?) their bodies.

Hadrian's Wall: 

(Vallum Hadriani) was a defensive fortification built by the Roman Empire to separate the province of Britannia from the northern lands of Caledonia. Construction begun in AD 122 during the reign of Emperor Hadrian and ran from the River Tyne near the North Sea to the Solway Firth on the Irish Sea.

The very popular Hadrian's Wall Footpath runs closely along the wall.  Guide books and handset recorded guides and even real live people guides are easiy available wherever you might be along the Wall, but many of each kind of guide really have too much information.  At 84 miles, it's nowhere near as long as the Appalachian trail (1293 miles), but I'd still recommend doing it in stages.

https://en.wikipedia.org/wiki/Hadrian%27s_Wall
There are many, no, more than many, books, of various accuracy and opinion about Hadrian's Wall, but, unless you are really Obsessive/Compulsive, they all seem to have too much information.  The Wikipedia  web site, link above, has a nice precis. 

A precis of the precis:  Started in 122 AD , during Hadrian's reign, and runs 84 miles long across Britania, from River Tine at North Sea end (Wallsend-Firth of Forth) to Bowness on Solway at the Irish Sea end.  Stone of turf wall (varied with time and section), V shaped ditch to its north, a military road and the Vallum, an earthwork ditch with parallel mounds to its south, the entire langth studded garrisoned forts, mile castles, and turrets (no position out of sight from the next). 

The garrisons were originally expected to be formed and maintained at locations behind the Wall, but, while the system was still under construction, it was decided to move their forts right up to the Wall.  Traffic and trade with the Caledonians north of the wall was controlled by Roman garrison troops at defended north facing gates of their forts along the Wall.

The following Internet link will connect with an intractive map with links to individual fort positions, now mostly just foundation walls, along Hadrian's Wall --  https://www.scribblemaps.com/maps/view/Hadrians_Wall_2/KY_K7nwjTy
Hadrian came to power in 117 AD succeeding the expansionist Trajan, who had grown the Empire to its greatest ever extent.  Hadrian realized that with the forces he had available to him, Trajan's "mega-empire" was indefensible and therefore he had to retreat to straighter defensible borders (the Latin word was limes, pl. limites).  When the well traveled Hadrian visited Britain in 122, he already had a plan to build  stone-wall limites across the island.  Within 10 years,  his plan was complete and manned by 10,000 allied auxiliary infantry (some mounted) in 17 forts, 80 mile castles, and 160 watch towers ("turets").

But in 142, Hadrian's successor, Antoninus Pius, had a different idea.  But wait....
 
Hadrian


Antoninus Pius

His wall across Britania was actually only a part of Hadrian's plan to stop the expansion of Roman sovereignty into new territory and, in some areas to actually pull back into more easily defended positions. 

In continental Europe, where wood was the available building material, he erected strong wooden defensive walls and garrisoned forts.  In Britania, his wall and the forts, milecastles, and towers were built in stone, although piled turf was initially used in western areas for quicker construction -- later replaced with stone. 

Building Hadrian's Wall by the Legions (but manned by allies) took several years and several design changes were made during that time (including locating the garrisoned forts right at the wall). 

Each segment of the defenses (continental or Britannic) was called in Latin a lime and multiple segments or the whole system would be called limites (lime, limites (pl) was the Latin word that meant "boundary".)


Before Hadrian's Wall:
During the reigns of the three emperors of the Flavian Dynasty (Vespasian and his two sons, Titus and Domitian (69 - 96 AD)) forces led by governor  Agricola built up a network of forts and roads in southern Caledonia (Scotland) to control access to the Highlands. 

This map shows the situation in 84 AD including a row of forts close to its bottom, on the Stanegate Road, just above the word, BRIGANTE, which is the name of the Celtic tribe that lived there. 

About a mile north of that row of forts is the line where Hadrian's Wall was built starting in 122
AD.  The forts in that row were intended to be expanded to house the troops who would man the wall.  But as we have seen, during construction, plans changed, and the troops were garrisoned in new forts built about every four miles right at the wall. 

Once Hadrian's wall was completed  the forts north of the wall were either abandoned or destroyed to prevent their use by the Caledonians.
 
One of the Stanegate Road forts, the one 5th from the left, right where two roads go off to the north, was Vindolanda, a mile or so south of the wall, that fort was expanded and became a supply depot for the wall.

The aerial photo shows the excavations at the Vindolanda site. The very regular rectangular upper part of the image is the dig of the standard plan military fort, and the much less regular lower part is the Vindolanda vicus, a civilian "camp follower" and sometimes "recreational" community that grew up right outside its gate in a manner commonly known even today next to long term military garrison camps. 

Among the most famous (and popularizing) finds at Vindolanda have been thin postcard size wooden writing tablets, which were written on with carbon based ink and were used for official and private letters.  The first few came to light in 1973, and, as of 2300, more than 1700 had been found.  One of them is an invitation to a 100 AD birthday party, the main text of which was written by a professional scribe,  but which also contains a personal note that may be the oldest surviving Latin language hand writing by a woman.

The letter shown is official and concerns ordered supplies of wheat, hides, and sinews that are late in their delivery.

Most of the Vindolanda tablets are in the British Museum in London, but some are planned to be put on display at Vindolanda.  They are almost all written in "old Latin cursive" which was the precursur of a later cursive style that ligatured the letters.  An informative Internet site on the Vindolanda Tablets is at https://en.wikipedia.org/wiki/Vindolanda_tablets.
  

   
The tablets and other organic artifacts (many leather and fabric) on display at  Vindolanda were preserved due to the water saturated de-oxygenated soil in which they were found.

The Vindolanda camp, vicus, and museum are open to visitors.  Summer excavation seasons continue annually.
Volunteer at Vindolanda --
https://www.vindolanda.com/pages/category/volunteering

Hadrian's Wall

English Heritage Internet site for the wall:
https://www.english-heritage.org.uk/visit/places/hadrians-wall/hadrians-wall-history-and-stories/history

Wall specifications:

Length: 80 Roman miles ( = approx. 73 statute miles = 117 Km)

Garrisoned Forts: 17 forts about 7 miles apart, manned by about 10,000 men total, almost all auxiliaries.  Years later, additional forts were built trailing southward down the coasts from both ends of the wall, River Tyne and Solway Firth.

Mile Castles: 80, with 10 to 30 men each, who patrolled northward and ran customs and migration.

Wall Height: nominally 15 feet ( = 4.6 meters, could vary with terrain or other circumstances).

Width: Started in the east at 10 feet wide, later reduced to 8 feet to save materials (started = 3 meters, reduced to 2.4 meters).

Walkway on full length.

Eastern end: Wallsend, River Tyne.

Western end: Solway Firth.

Garrison: Foreign allied auxiliary infantry.

Building period: Finished within ten years following 122 AD.  Continuous maintenance while manned.

An aproximately 40 Km turf section (6 meters wide) at the western end was later redone in stone.  Mile castles and turrets on that section were built in stone from the beginning along the turf wall.  Three forts were left north of the turf wall.  Supplies on the turf section came from pre-existing Stanegate forts south of the wall.
Wall profile:  North of the wall (left) a 20 foot wide sloping berm and a ditch 30 feet wide and 13.5 feet deep. the dirt from the ditch would be heapd up between the dith and the wall to serve as a 60 foot glacis leading up to the wall.  Then came the wall itself,15 feet high above the glacis.  Every 1/3 mile along the wall's full 80 Roman miles would eithr be a Mile Castle or a Turet, all of which were manned and patrolled.

Mile Castles, one shown, would have 30 to 50 men.  A Turet gate on the north end of Mile Castles would control movement of persons and goods (customs and immigration). 

South of the wall was the Valum consisting of a wide ditch with a 20 foot wide flat bottom and mounds on both sided composed of the the earth dug from the ditch

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One of the largest forts, right behind the wall:
Originally it was thought the forts would be at some distance behind the wall, perhaps as far as the Stanegate forts, one mile back, like Vindoland.  But, while construction of the wall and forts was already under way, a decision was made to move the forts right up adjacent to, but not built into the fabric of the wall. 

At about the midpoint of Hadrian's Wall, the excavations at Housesteads Fort have revealed what is said to be the most complete Roman fort in Britain and one of the best known anywhere in the empire.  Completed within ten years of the start of Hadrian's Wall in 122 AD, it had a garrison of 800 foreign infantry.  The same unit (obviously with replacements) held the fort until the end of the 4th century. 

A turret (tower), at first outside the fort walls, was later included during an expansion. 

The fort was manned by "the first cohort of Tungrians" that was detached from Vindolanda about a mile south of the wall.  The Tungrian auxiliaries were auxiliaries originally recruited from German speaking tribes from the Tongres district in modern Belgium.  Where replacements came from over the years is unknown, but the unit kept its same name throughout its tenure.

A civilian settlement (vicus) developed outside the fort.
Like at many other installations on the wall, the Housesteads Fort completion was the occasion for an inscribed dedication including its completion date, which Legion built it, and where a unit came from who might garrison the position, most likely foreign auxiliaries (the garrison being other than the Legion that built the instalations.)
Housesteads Fort


The rectangular multi-chambered feature
 in the detail image is the residence of the fort's Tungrian commander and his family, inside the walls of Housesteads Fort. 

The three red-roofed buildings in the visualization of the Housesteads Fort and vicus, below, are the three building, the runs of which are shown exposed in the detail image above.


Various reports indicate that 600 to 800 men manned the Fort and detached men to the closest Mile Csstles and Turets.
A modern drawing of a typical Mile Castle, which, as the name states, were built one mile apart between garrisoned forts on the wall.  The ruins of various mile castles indicate that the various military units that built the emplacements and associated sections of the wall were working from standardized plans, although variation was allowed to take into account terrain and other local requirements.  The northern, or turet, gate would be where persons and goods crossing the wall would be controlled, and the southern gate was where the Garrison's supplies would be delivered from a supply depot further south via a road across the Vallum.






Brunton Turret, now counted as 26B, imagined reconstruction drawing, a stone tower like many others with a (conjectured) observation walkway at the top level.

A little more than the bottom eight feet of the tower is preserved today. 

Turrets were nominally one third of a mile apart and built right into the wall, so there would be two between each adjacent pair of Mile Castles (plus a turret at some Mile Castles.  Along its 80 mile length there were about 160 Turrets.  Turrets had one back (south) door and no opening to the north.

Three to 8 men at a time swould be expected to man each Turret, on duty rosters drawn from among the groups of about30 to 50 soldiers that would man adjacent Mile Castle who were, in turn, drawn from the large 500 men or larger   nominal seventeen Fort garrisons.  The Roman military, like its modern counterpart appear to have loved making duty rosters.

Using a numbering system introduced in 1930, Mile Castles are numbered 1 to 80 from east to west. Each pair of turrets takes the number of the nearest Mile Castle to the east, ‘a’ for the eastern turret and ‘b’ for the western one.

Jumping forward:  410 AD -- all the Roman troops and and bureaucrats essential to manning forts and walls, towns and cities either withdrawn for more pressing duties on the continent or chased away, leaving Hadrian's Wall to become a quarry of pre-cut stones for later farms and municipalities.
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The Antonine Wall
Hadrian's "limites", i.e., boundaries, program had not been popular with the moneyed classes of Rome.  The Senators wanted military commands and the commercially oriented equites (read, knights) wanted both jobs and trade opportunities that came with military expansion (and that did not come with military units sitting quietly on fixed, straight, retracted borders).  Reasoning with them about border-duty costing less (and requiring lower tax rates) just did not win -- especially since taxes never seemed to really decrease.  The military-industrial-complex needed expansive wars. 

Whether all that, or some now not known incident(s) involving the Caledonii on the island, or, very likely, because the Antoninus Pius regime, after succeeding Hadrian, just needed something military to justify and validate its existence -- whatever the reason, Antoninus Pius ordered action. 
Legions marched northward from Hadrian's line in northern Britain into Caledonian territory in what is now Scotland, into the Lowlands.  There a new defensive line would be drawn and a new wall would be built -- and, because it would be about half as long, it should cost only half as much in men and money.

So, in 142 AD, just 20 hears after the start and 10 years after the completion of Hadrian's limites a new construction project was begun 100 to 115 miles to the north near the middle of the Caledonian (Scottish) lowlands.
 
Construction of the Antonine Wall between the Firth of Clyde and the Firth of Forth (39 miles, 63 kilometers) took, by some estimates, as much as 12
years.

<--- The wall, piled sods on a stone foundation, had 16 forts with small "fortlets" between them. 



The Antonine Wall was, in most of its length, a 10 foot (three meters) high and 16 foot (7+ meters) wide bank of stacked turves with more turves sloping up from the ditch on the northern side. 

Into the slope were dug rows of pits called lilia (plural of lilium), offset from each other, hidden by light brushwood, and lined with sharpened stakes, to serve as obstacles to slow or stop advancing enemies moving toward the wall's north face. 

On top of the turf wall was a stout wooden fortification surmounted by a walkway behind a palisade, along which defenders could move to face an attack. 

Right behind the wall were the stone forts and smaller fortlets which had what are now characterized as "wing walls", sections of stone built wall extendingt a bit along the turf wall.  (The wing walls are thought to be evidence of original intention to rebuild the whole wall in stone -- but never carried out.)

Unlike Hadrian's Wall, the Antonine Wall did not have the complex of mounds and wide flat-bottomed deep ditch on its southern side but had just the "Military Way" road on that side for quick movement of troops if needed.  The troops that manned the Antonine Wall were simply drawn forward from Hadrian's Wall.

Antonine Wall forts were originally planned to be 6 miles apart, but that was quickly revised downward to 2 miles apart with intermediate fortlets. 

The best preserved of the forts, but also the second smallest
         https://en.wikipedia.org/wiki/Rough_Castle_Fort
Inscriptions found at the fort show that it was garisonned by a detachment of 480 alied Celtic auxiliary infantrymen, part of the VI cohort Nerviorum that had been originally been recruited in northern Gaul.  (The main section of the cohort is recorded to hace been at the Grantchester Fort on Hadrian's Wall from the 2nd through 4th centuries.)
 Rough Castle Fort (visualization drawing), built up at the highest point on the Antonine Wall
 
Re-excavated Lilia pits north of  Rough Castle Fort.



The Lilia pits are also visible on the left edge of the adjacent image (barely visible) as rows of tiny dots at the level of the
orange tile roofed administration buildings inside the fort. Only the HQ buildings were built in stone.
The Antonine Wall was abandoned by the Romans in the 160s AD, only about 20 years after it was built, and the garrisons moved back to their previous quarters at Hadrian's wall.  It may be that the more northern position was harder to supply, or that it was competing for troops with more important places (e.g. Germania in mainland Europe), or that the money and effort spent was just too much. 

At any rate, Antoninus Pius, the sponsor of the Antonine Wall, died in 160 AD and his two adopted sons, Marcus Aurelius and Lucius Verus, who ruled jointly until 169 when Verus died (Antonine Plague ?) neither pursued nor funded continuing the wall project.

As those of us who were in the military often heard, "It was a headquarters decision" or "It was way above our rank" or "It happened during the Change of Command".
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If you visit the Antonine Wall today, what you will find in all but a few places is something less endearing than the Bar Hill Fort, Scotland.  Even at Bar Hill there is little more leeft than foundation stones of the fort's HQ and the bath outside fort wall, everything else having been taken away as spolia to be used in later construction. A few dedicatory and memorial stones and other artifacts were found. 


Remains of the Antonine Wall near the Bar Hill Fort.  The south, Roman side of the wall is to the leftof this image. 

The downward slope to the right is into the ditch which hopefully would slow red-haired, tartan-wearing, attacking-from-the-north Caledonii.