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 Islands 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.

Agricola's Marching Forts

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 easily 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 or 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 with 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 interactive 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

Hadrian's wall across Britania was actually only a part of his plan to stop the expansion of Roman sovereignty into any 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. 

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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 2023 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: 84 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 ruins 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.