Attack Priority

I'm having an issue in my map that I can't seem to fix. I thought the stats - priority field would handle it, but apparently that's for their icon position in a list of multiple units being selected.

My issue is that, admist a great battle, the siege units are always attacked last. For instance (and this definitely happened a number of times), a number of siege units will be center map. I'll send melee units to attack them from one side, and another enemy will attack with melee units on teh other side. Instead of us both attacking the siege units in either side, our units will sort of run into the middle of the group of siege units and attack each other. In teh end, the siege units are left standing with the rest of us dead!

Sounds more like an AI problem to me, do the siege units belong to the same player as the other attackers, or do they have their own player?

In the example above, there were 3 players involved -- 2 had melee units and the other had siege.