Actually aliens already attack towers, but NPC ones, in sector 35 apf.
But the idea is really lovely, if alien levels will be high enough - it can provide good challenge and could enlarge tl7 pvp on all servers because such alien attacks can greatly help (if alien levels will be pretty high) opposite factions to get tower fields to themselves.
Aswell it can provide some unique loot from unique aliens during tower attacks.
This idea is really good and can be greatly improved with new sub-ideas.