OK, here's what happened:
Was fighting off Gallic raids in the north, and one of my cities had a full stack headed at it. Had nothing to stop it in the field (busy fighting other armies) so I sent 4 units of archers to help them hold it. BIG MISTAKE.
Evidently, the Senate is incredibly trigger-happy, and it proceeded to attack my archers with Praetorians. My men got massacred and I was at war with the Senate. I recruited a massive merc army and sieged Rome. Took it because forgot to immediately activate garrison script when I started back up just after starting siege. Anyway, Rome's gone, but the Praetorian cav were still hanging around. Killed them.

It still says the SPQR exists.......which doesn't make sense because they're all dead. Anyway, is this because the garrison script wasn't activated all the way, or is it something else?