Hadrian's Villa site features

The site covers at least a square kilometre -- more ground than the city of Pompeii -- and it is still only partly excavated. Over thirty monumental buildings sit on a series of artificial esplanades cut and built at different heights, with gardens, water basins and nymphaea between them. Because the levels are artificial, the walk is a sequence of terraces rather than a single plain, and the drops between them are held up by substructures that are themselves buildings.

What you walk into first

The Poecile is the largest single space: a rectangular pool 232 by 97 metres inside four walls with a colonnade on the inner face, raised on massive walls to make a level platform with windows looking out over the country. Its northern side was a double portico whose central spine wall still stands 9 metres high -- cylindrical shrubs are planted where the roof columns were -- and the circuit round that wall measures 430 metres, laid out so that it could be walked or run in bad weather. Underneath, on the south and west, the Cento Camerelle bridges a drop of up to 15 metres: rows of identical rooms up to four storeys high, separated by the buttresses that carry the terrace, reached from external wooden balconies because cutting doors through the buttresses would have weakened them.

The set pieces

The Maritime Theatre is a circle 40 metres across with its entrance on the north: a ring portico of unfluted Ionic columns under a barrel vault, then a ring-shaped pool paved in marble, then an island 25 metres wide in the middle of it. On the island is a complete miniature house -- atrium with portico and small garden, library, triclinium, baths, rooms arranged round a peristyle with four fronts curving inward, a fountain at the centre. Two wooden drawbridges were the only way across. One of the bath rooms has a tub with steps down into the moat, which was probably used for swimming.

The Canopus is named for the Egyptian city and the stretch of Nile that runs to it, and it is built as both: a long reflecting pool standing for the river, with an open-air dining area at the end of it under a covered triclinium holding a huge semicircular masonry couch, set inside an enormous domed exedra looking back down the water. Its sculpture is the most complete on the site -- copies of the caryatids of the Erechtheion, a figure of the Egyptian dwarf god Bes, a crocodile.

The Piazza d'Oro is a peristyle 61 by 51 metres round a garden with a central canal, flowerbeds and pools, its portico split into two aisles by alternating columns of cipollino and Egyptian green granite. You enter from the north under an "umbrella" dome; the south side curves through a sequence of concave and convex rooms to an octagonal exedra under a cavernous dome, and ends at a semicircular nymphaeum whose seven alternating niches once poured water into a pool.

The Palazzo d'Inverno stands on three levels over a hypocaust system. Its third level is a courtyard of forty columns, 59 by 33.5 metres, with a pool 28 metres long down the middle -- once faced in white marble, set in mosaic paving, and placed to throw light and reflection into the rooms that overlook the Poecile.

The Maritime Theatre

One of the first buildings begun, in 118. A miniature house on an artificial island inside a circular moat, the whole enclosure 40 metres across with the island 25 metres wide, linked to the ring portico by two wooden drawbridges. The name comes from the marble friezes of tritons, nereids and cupids in chariots drawn by animals, not from any theatre. Its plan is symmetrical throughout, which is rare in Roman architecture, and it does structural work in the layout as a whole by joining the Imperial Palace, the baths and the Porticus Miliaria, which sit on different alignments. It has been read as Hadrian's first residence on the site while the rest went up.

The Canopus and its Serapeum

A long reflecting pool standing in for the Nile, closed at one end by a domed exedra over a covered triclinium with a semicircular masonry couch. Among the copies set around it are the caryatids of the Erechtheion, the Egyptian dwarf and fertility god Bes, and a crocodile.

The Poecile

Modelled on the Stoa Poikile in the agora at Athens, the city Hadrian liked best of everywhere he travelled. A pool 232 by 97 metres in a walled, colonnaded enclosure on a raised platform, with a double portico to the north whose 9-metre spine wall survives entire; the circuit around it is 430 metres, built for walking or running under cover.

The Cento Camerelle

The substructure that makes the Poecile possible, taking up a height difference of as much as 15 metres on the south and west. Identical rooms, up to four floors of them, divided by the buttresses that hold the terrace, with wooden floors, one opening each and external wooden balconies instead of doors through the structure. The plainness of the finishes, the number of rooms, the many latrines and the service road running underground into the vestibule all point to the humblest servants or slaves, or the imperial guard.

The Hall of Philosophers

Between the Poecile and the Maritime Theatre, entered on the short north side through four columns under a trabeation, with a hemispherically domed apse at the south end holding seven rectangular niches -- probably for the seven philosophers or the seven sages of Greece. Marble floor, walls lined in alabaster, marble and above all red porphyry, the imperial stone. Most likely the throne room for official audiences.

The Baths with Heliocaminus

Hadrian's personal baths, begun around 118, containing the innovation the complex is named for: a room heated both by sunlight and by a hypocaust, under a coffered dome with an opening at its centre and large windows facing south-west.

The Hospitalia

A dormitory for high-ranking staff: a long central room with five rooms down each side, each on a T-shaped plan making three alcoves for beds. The floors are the reason to stand in it -- monochrome mosaics in a range of elaborate floral patterns typical of the second century. A hall at the end carries a statue base and was probably a small temple.

The cryptoporticus with the mosaic vault

Cut into the tufa under the atrium of the Republican villa that Hadrian built over, a rectangle of barrel-vaulted galleries lit by windows. The vault of its south-east gallery carries what is probably the oldest surviving vault mosaic anywhere, from the mid-1st century B.C. -- older than the emperor's villa by nearly two hundred years.

The Antinoeion

Rediscovered in 1998 on the main road to the grand vestibule, where a large concrete foundation marks the original position of the Antinous Obelisk now standing on the Pincian Hill in Rome. Whether it was Antinous's tomb or a heavily Egyptianising nymphaeum is disputed, and the argument is unresolved.