I was informed this is the Apodyterion (= where people took off their cloths) of Faustina's Baths.

The Bathes of Faustina, 2nd century AD, founded by Faustina. She was the spouse of emperor Marcus Aulerius. She gave birth to 12 children and accompanied her husband as "mater castrorum" on his many travels. She was named (after her death) "pia" (pious), partly for her many foundations.