Its not just bats, its any "invasion". Defenses don't matter, the mobs will spawn where ever you are located. I have lost my pigs 3xs to this issue now (bad timing of an invasion spawning right as I'm harvesting my veggies or feeding my pigs), its getting rediculous. What is the point of offering defensive buildings, if they are just gonna be useless. Mobs should NOT be able to spawn INSIDE your base. It makes no sense at all!