How to prevent intruders into apartment building

[Stone Teran]

I purchased a 62-unit apartment building with senior housing 7 months ago.  No matter what I try, I can't keep intruders out.  They sleep in the halls, piss in the corners and there's some prostitution.  The two main points of entry are the main door (keyed single door with panic bar on inside) and back door (unkeyed double doors with panic bar on inside).  The main door is public and not really a problem.  The back door is where my battle is.  Previous intruders and tenants are damaging my panic bar, taping it shut, and wedging stuff to keep it open.  I'm already on eviction #3 of main tenant suspects.  The fire department is heavy handed here and won't let me lock the door or remove the panic bar.  I'd like to keep costs down and I'm open to all sort of ideas.