Ah! YES - I recognise this place!! Like you, I also stumbled across it unexpectedly whilst meandering about the streets of Rome. 🤩 ‘La Citta del Aqua’ (“The City of Water”). When I was there back in 2012, I recall it not being so busy - a welcome escape from the overcrowded streets above. We didn’t have to book in advance and had the ruins to ourselves for an hour or so. A hidden gem. One of the lesser-known secrets of Roma.