Perhaps make a system where for 20 BP you get a "Barricade Maker" which creates tentacles that protect nearby buildings. Once you deal X amount of damage to the building though, it has to stop and regenerate more barriacades. So it acts as a temporary invulnerability shield, and either forces humans to stop and look for it, or at least holds them off for a bit, giving aliens time to respond.
Since "Hold them off long enough for us to defend better" is the general idea of a barricade...
(To clarify, the tentacles it spawns wouldn't be actual pysical objects, it'd just be an animation to show that the building is protected from a certain amount of damage. In other words, having a barricades in your base would just add 100-200 hp to EVERYTHING, so you can have a better chance of surviving.)