Just east of the Roman Colosseum are the remains of the largest barracks for gladiators in Rome: the Ludus Magnus. The building is of brick-faced concrete throughout and the building was started by emperor Domitian and completed some time during the reign of emperor Hadrian.

Its precise location was uncertain until 1937, although its layout was partially known from the Forma Urbis Romae (a marble map of Rome from the era of Septimius Severus, which has come down to us in fragments) . Begun in 1937 and continued between 1957 and 1961, the excavations brought to light only the north section of the building, but looking at it one can easily imagine the curved structure of the cavea. With the help of a fragment of the Forma Urbis on which the name of the building appears, it is possible to give a precise and complete description of the complex.

It is thought that the building was probably three floors high with up to 145 rooms altogether. Ludus Magnus consisted of an elliptical arena with a long axis of 62 m and a short one of 45 m surrounded by the stands of a small cavea originally faced with marble slabs. The cavea was large enough to hold nine terraces and had room for approximately 2500 spectators.

The main entry to the arena were located on the long axis, while there were boxes for public authorities on the short one. The courtyard functioned like an amphitheatre and this was where the gladiators did their training.

On the north side of the excavated area, facing Via Labicana, one can see a fairly well conserved row of little cells provided with stairs leading to the upper floors. The warriors lived in the Ludus in a permanent state of captivity and were subjected to a severe program of everyday training.

A tunnel connected the arena directly to the east entrance of the Colosseum. The original construction dates back to the era of Domitian and must have caused the demolition of a residential neighbourhood of the late-republican and Augustan periods, of which there is clear evidence (the remains of a tessellated floor can be seen on the south side of the area, in the direction of the Celian hill). The remains of the cavea and the arena, on the other hand, belong to a restoration carried out under Trajan. There must have been other buildings similar to the Ludus Magnus overlooking the square: the Ludus Matutinus, where the venatores were trained, and the Ludus Dacicus and Ludus Gallicus, which were named after the gladiators who lived there.

We must also imagine in the immediate vicinity all the auxiliary buildings we know were connected with the Colosseum, such as the spoliarium (where the corpses were collected after the fights in the arena), the samarium (where wounded gladiators were taken), and the armamentarium (where weapons were stored). Probably further north, in addition, there were the Castra Misenatium, where the sailors in charge of the velarium lived, and the Summum Choragium, where the machinery used in staging the games was stored.