Your tower looks really secure where I can’t find a good reason why any of those guys that are on floor two and above are dying. NPCs on first floor can start to wander and can get into trouble.
Next time someone dies, take note of where you are and any hazards that might be present (like is it nighttime, you’re nearby and wraiths could’ve made their way up there, or did you ge die and a Wyvern might’ve followed you, was it a blood moon and curious zombos are opening doors, etc).
If it continues where you don’t have a reasonable explanation of them dying, once you have a couple instances with some notes, send it in to Pipeworks here:
https://fs30.formsite.com/505games/form15/index.html