For apartment security – what floor are you on? if you’re above ground floor without a fire-escape you probably don’t need to worry about windows (if you’re on ground floor, there are security films that *might* be removable… but I’d be wary of violating the lease for anything permanent).
There are also portable door-jammers, read the reviews and be careful… apparently some of these are so strong that deploying them can actually damage your door (like, destroy the door knob…) if used improperly but there are many options:
You might also consider security cameras. I’m still looking for a good system, but for example Arlo security cameras are wireless and battery powered. You might be able to find a way to set one up without it being stolen (semi-public halfway access I assume – maybe you could hide it in a “decoration” or bush/tree outside?) or without involving a permanent fixture. Be aware – these tend to require an active internet connection (and power) to be useful.
You can also think about creative use of furniture… do you have book cases or dressers strategically placed near doors or windows that can be moved into a blocking position without too much effort? Are there “tools” that might double as home defense without running afoul of your lease agreement?