It is a quadrifrons trumphal arch, surmounted by an unusual octagonal cupola,and was erected (entirely in marble) by Gaius Calpurnius Celsus, quinquennial duumvir of the city, to commemorate the victories of Lucius Verus, junior colleague and adoptive brother of the Emperor Marcus Aurelius, over the Parthians in the Roman–Parthian War of 161–66.