Basilica of Maxentius/Constantine: The biggest single structure still standing in the Roman Forum is the remaining part of the Basilica of Maxentius, also known as the Basilica of Constantine. It has a double name, because it was one of the many projects started by Maxentius that were completed by Constantine after Constantine trashed co-Emperor Maxentius at the Battle of the Milvian bridge in 312 AD. The Basilica was the last non-Christian building built in the Roman Forum -- all that followed were churches, and most of those were built into the remains of roman civic structures or temples. Like the earlier basilicas, this one was built to provide space for political and commercial wheeling and dealing. It is likely that it also was used, as were the others, as a court for civil law cases: the magistrate (perhaps even the Emperor, although he soon moved his venue to Constantinople) would have sat in one of the apses.

Before meeting his watery end in the Tiber, Maxentius had started building (408 AD) his great Basilica on the north side of the Via Sacra (Sacred Way) in the Republican forum. He had chosen a location that required the leveling of part of the low Velia ridge that had separated the Colosseum from the Forum. Constantine completed the Basilica with some modifications, the most important of which was a new grand entrance in the center of the long southern side and an apse in the center of the northern side just opposite his new entrance. Some authorities say Constantine also added the twenty-meter wide apse at the western end of the central nave, opposite Maxentius' entrance, but he just may have completed what Maxentius had already started.

Also in dispute is where the ten-meter-tall statue of Constantine was placed. There are early writings that place the Constantine statue in each of the two apses. One reasonable conjecture is that the statue of Constantine on his curile throne, along with a high and massive base, may have been moved from one apse to the other and that different visitors saw it at different places within the Basilica. At any rate, the fragments now on display in the Palazzo dei Conservatori on the Campidoglio were said to have been found in the western apse in 1487. (For comparison, the statue of Lincoln in the Lincoln Memorial in Washington, D.C., on the same kind of throne, is six meters high.)

The easiest way to find the Basilica today is to head north from the Arch of Titus at the east end of the Forum, jogging left around the western end of the Santa Maria Nova Church. Follow the paths through the screen of trees and it will jump out at you: it's too big to miss. What you see when you get there (or, if you take the best longer view, what you can see from between the twin pavilions of the Vignola aviary in the Farnese Gardens on the Palatine) looks like three giant, vaulted, side-by-side structures built into one even more giant building. And these are actually just side vaults that formed an aisle on the north side of a truly much bigger central nave. There were three equivalent vaults, another aisle, on the south side of the nave, but they and the central nave, probably collapsed in the great earthquake of 847 AD.

We're talking about a really big building here. The platform on which it was built is solid concrete 100 meters long and 65 meters wide and nobody knows how thick. The central nave was a single open space 80 meters long, 25 meters wide, and 35 meters high. The side aisles, each with their three barrel-vaults, were 16 meters wide and 24.5 meters high. The skeleton of the Basilica was good late-Roman concrete faced with brick. Although the outside was plain and perhaps never sheathed in stone, the inside was sumptuously decorated with carved and colored architectural marble and statuary. In front of the piers supporting the central and side vaults were eight marble monolithic-shaft columns. (The only one that survives was moved to Piazza di S. Maria Maggiore in 1613, where it still stands.) The ceiling was decorated with hexagonal and octagonal coffers and was probably modeled on the inside of Hadrian's Pantheon dome. The gilded bronze tiles, which covered the roof, were reused on the first Saint Peter's Basilica.

Like so many other buildings in Rome, this one features in many Internet sites, but this one you need to see in person. If you can't get there, here are the links:
 
 

Description of the Basilica from the Platner and Ashby Topographical Dictionary:
http://www.ukans.edu/history/index/europe/ancient_rome/E/Gazetteer/Places/Europe/Italy/Lazio/Roma/Rome/.Texts/PLATOP*/Basilica_Constantini.html

A short description with cross links:
http://www.seindal.dk/rene/sights/sight/177.htm

The Constantine Statue:
http://www.bluffton.edu/~sullivanm/romanconstantine/constantine.html

US National Park Service Lincoln Memorial web site (with a link to the Lincoln Statue site):
http://www.nps.gov/linc/index2.htm