Spell it, outside and inside.
Who conquered what? (See Gordian's Notebook.)
Regia/Shields (a.d. XIV Kal. Nov.)
The Acta and scrolls report tampering with the sacred shields of Mars at the Regia. Enter the Regia (near the Temple of Vesta). On the floor to the left is a circular inscription; click on the "capstone" you found earlier. This opens the courtyard door. There are three circles on the ground here; each has three rings. Move the rings on each circle so that each forms a letter: PAX. (There's a fun animation on one of the blank niches here, by the way; move your cursor over the figures to see them twirl.) This process opens a second door; enter it, open the cabinet, and solve the anagram puzzle: BELLUM ET PAX (war and peace). This opens yet another door, to the shield room. Hang the appropriate shield on each emperor's name (clue in Gordian's notebook on Emperors): Arabia on Severus, Gallia on Julius, Egypt on Augustus, Britannia on Claudius, Dacia on Trajan. A hole in the floor opens up. Go down and get the acorn, Regia tunnel key, and scroll. Open the door, and you'll find yourself in the House of the Vestal Virgins.