Short answers, pulled from the story.
The Altar of Domitius Ahenobarbus is a series of four sculpted marble plaques that originally formed the base of cult statues in a Temple of Neptune on the Field of Mars in Rome. One portion is on display at the Louvre in Paris (Ma 975) and another at the Glyptothek in Munich (Inv. 239); a copy of the Munich piece is held at the Pushkin Museum in Moscow.
The frieze is dated to the end of the second century BC, making it the second oldest Roman bas-relief currently known. The census panel on the Louvre portion is also one of the earliest examples of the continuous narrative style in Roman art.
The temple was built by Cn. Domitius Ahenobarbus, who attained the consulship in 122 BC. He had likely vowed to build a temple to Neptune after a naval victory off Samos in 129 or 128 BC against Aristonicus.
The four panels show two different subjects. The Louvre panel depicts a Roman census, including the recording of citizens, the suovetaurilia sacrifice to Mars, and a levy of soldiers. The three Munich panels depict the mythological wedding of Neptune and Amphitrite surrounded by Tritons, Nereids, and Erotes.
The reliefs were mentioned in records from 1629 and 1631, after being uncovered during construction work by the Roman Santacroce family between 1598 and 1641. By 1683 they were documented as decoration in a palace courtyard near the Tiber.
The suovetaurilia is a traditional Roman triple sacrifice consisting of a bull, a ram, and a pig, offered to Mars. On the census panel, this ceremony called the lustrum is presided over by one of the two censors and includes musicians playing the lyre and tibia alongside young priestly assistants.