There are local garrisons, as well as marines who serve on ships going into and out of Italian cities plus militias. The pope also maintains a force as a guard, and Italy does have decent fortifications around its cities. All together no one has gotten a solid plan to actually invade yet. That's not to say the Franks invading is going to be the first time Italy is going to be attacked/raided. We just haven't gotten to that point yet. The Vikings OTL raiding Italy in the 860s, and that's when Constantine VI comes to power. ITTL's Vikings aren't going to reach the peninsula for a while longer, as Italy is still far stronger than it was OTL during this time period, but they'll be showing up soon enough.Rich and weak, what could possibly go wrong? In that state I'm surprised it takes as long for someone to invade, why not some enterprising adventurers?