Porta Nomentana

Porta Nomentana, on Viale del Policlinico, was part of Aurelian's original circuit and was walled up in 1564, when Porta Pia opened, according to Italian Wikipedia, which also says the walled-up gate now forms part of the perimeter wall of the British Embassy compound. The embassy stands on Italian soil; diplomatic premises are inviolable under the Vienna Convention but are not foreign territory.