Museum of the Ara Pacis site features

The Ara Pacis is a white marble box standing inside a glass one. The monument is a central sacrificial altar enclosed by precinct walls of Luna marble, raised on a podium and entered from the west — and the whole thing sits in a daylit hall of glass and steel on the Tiber embankment, so you walk all the way round the outside of it before going in.

The outer walls carry two registers and they are easy to tell apart. The lower register is a vegetal frieze — a continuous band of acanthus scrolls filled with animals, dense and shallow-cut. Above it runs the famous part: a procession with members of the major priestly colleges, Roman magistrates carrying fasces, attendants, and identifiable members of the imperial family moving in one direction along the wall.

Step through the opening and the decoration changes register completely. The interior walls of the precinct carry a frieze of bucrania — ox skulls — alternating with garlands of fruit and flowers, with the lower part of the interior treated differently again. It is the inside of an enclosure rather than the outside of a building.

The altar itself, at the centre, is the most damaged thing here. It preserves fragments of scenes illustrating the lex arae, the rules governing the sacrifices performed on it.

One panel is worth finding on its own. A well-preserved mythological panel shows a seated female figure surrounded by children, vegetation and personifications of breezes — the most complete figural composition on the monument.

What you are looking at is a reconstruction against a deadline, and it shows. The altar was commissioned by the Roman Senate on 4 July 13 BC to honour Augustus's return, and it had stood on the north-eastern corner of the Campus Martius, on the city's northern outskirts, until it was buried under 4 metres of silt. Fragments surfaced in 1568 beneath a palazzo; Friedrich von Duhn identified the reliefs as belonging to the Ara Pacis between 1879 and 1881; excavation resumed in 1903. Because the reassembly had to be completed by 23 September 1938 and only limited ancient material and documentation survived, much of what holds the surviving carving together is modern. The joins are visible if you look for them.

The building around it is the second thing people argue about. In 1938 the reconstructed altar was placed near the Mausoleum of Augustus inside a pavilion by Vittorio Ballio Morpurgo — known locally as the teca del Morpurgo. That was pulled down in 2006 and replaced by the present glass and steel structure, Richard Meier's design, which caused new arguments and criticism from Roman inhabitants and foreign observers alike. Whatever you make of it, the light it throws on the marble is now part of the experience.

Practically: the museum is on Lungotevere in Augusta, open every day 9.30 to 19.30 with last admission an hour before closing, and the museum-only ticket is EUR 14.00, or EUR 8.50 concessions.

The processional friezes

The long north and south walls show the state ceremony for the altar's dedication. Among the participants are members of the major priestly colleges, magistrates carrying fasces, and identifiable members of the imperial household — Augustus, Agrippa, Livia, Tiberius and Julia. Women and children appear prominently, a notable change from earlier Roman public monuments, and scholars read the children as reinforcing Augustus's moral and family reforms.

The contested goddess panel

A seated female figure surrounded by children, vegetation and personifications of breezes, plants and animals, variously identified as Pax, Italia, Venus or Tellus. The disagreement is not a gap in the record — it is a live argument about what the altar is chiefly asserting.

The vegetal scrollwork

The lower register of the outer precinct wall: a continuous frieze of acanthus scrolls filled with animals, small birds, insects and mythological creatures, usually read as growth, abundance and the prosperity claimed for Augustus's rule. It is the most technically impressive carving on the monument and the easiest to walk past.

The plank carving inside

Step in and the inner face of the same wall is a different monument: bucrania — ox skulls — alternating with garlands, and a lower zone cut to imitate wooden planks. That imitation links the altar to the older altars set at the boundaries of Rome's pomerium, which is a claim to ancestral practice rather than a decorative choice. Look for planks on the outside and you will only find vines.

The lex arae fragments

On the central altar, scenes setting out the rules for the sacrifices performed here, with attendants leading animals — the working instructions of the monument rather than its message.

Richard Meier's pavilion

The 2006 building that encloses the Ara Pacis: a glass and steel structure on the site of Mussolini's shelter, considerably larger, and divided into several rooms beyond the altar hall. Free to look at from the Lungotevere and from Piazza Augusto Imperatore.