(please add to this)
Bases can be bought for varing sums of cash. Different shapes and locations, including the exterior building govern your desisions when the need to expand. Floor plans are number one and extorier shape is number two for importance. A weak high rise slum can collaspe and destroy your base module, whereas a fortified wharehouse surreounded by high rise buildings can offer the best "projectile damage resistense" because its not easy to shoot at.