Intresting concept, sure would be fun for a co-op game! :)
I tried it a little and this is just my 5 cents:
+I like the starting base being this airfield in the town, it feels meaningful and important to defend that place.
-A bit too easy in general since the 'enemy' has to get up through the narrow paths to then attack the town, perhaps make more routes to exit thier base? I'd prefer it when it's a bit less predictable where they come from forcing the defending team to hold back in the town in the early game, then in late game you can take the risk of advancing out.