In the original X-COM game, the bases were located underground, since the first thing you had to build was the access lift. As such, the only way for aliens to assault your base was through the access lift or the hangars.
But in Xenonauts, the aliens seem to be able to just attack from wherever, firing breaching pods into the side of your base or something. This raise the question, though, of why they even bother to send in troops in the first place? If they have a giant ship, why don't they just glass the base from the sky?