its on every map that has more than one level... ie barns and buildings with 2 or more levels. When i click to go up the stairs the actor will climb the stairs then when i click for him to walk across the floor ect he ends up back on level 1.
As far as the base attack by aliens i have only had one base attack, it was on my main base. So i cannot say if its that specific map or not, with the aliens attacking the base.