A couple come to mind - Antioch, Kiev, Jerusalem, Pamplona, Antwerp.

Antioch has the Eggys and the Turks fighting over it. When one of them wins, the Mongols make a beeline for it too. Kiev has Russkies, Poles, and Hungarians stampeding for it. Jerusalem is a Crusader's timeshare. Pamplona is always under French or English attack while you are trying to complete the Reconquista. Antwerp is another hotspot like Bruges, simply on the path of expansion for a lot of factions.

Other than that, I think most everywhere else is only seiged a bit before the front moves past them. In my games I find Northern Italy entirely too stable. One faction takes it over, and that's it until I come through.