Aphistamenoi: who are these jerks?

Alright, so I am playing a Rome campaign on Very Hard, and these guys (rebels of somesort) keep showing up once or twicece a year with rather powerful armies in the understaffed middle of Italy (I am not very far). Who are they, and where do they come from? I assume they are being randomly spawned, because my borders are pretty well defended and they appear all of a sudden.

So, anyway to deal with these guys other then forming another army every time they show up? Do they have a source? Will strengthening my city garrisons help?

oop, wrong section. sorry.

Did you build watchtowers there? I believe they spawn at unguarded watchtowers.

??? Hax seems to know what he´s talking about, so I´ll just assume everything is fine.

Arent watchtowers a bit to expensive in the early game to be build in quantity?