As a contractor, I would say this. IF the BG has any sense, he can get into almost any house in a minute or less.
That said, there are many things to do to make your place harder.
There are also many things that you can do that will probably violate your lease and end up costing you money to get "repaired".
The simple chain across the door is good. cheap, strong and usually can be "repaired" at move out with a little spackle on the holes. Instead of a biner, just get a chain hook on a big lag bolt. Have the lag be on the hinge side.
A big chunk of 2x4 or 4x4 laid behind the door preventing swing is fool proof and easy to move if you need to escape a fire. If you have a double tumbler lock installed and you have a fire and someone gets hurt in your place you may be liable. I Always make sure we leave one Key chained to a closet or similar, that is out of sight and yet instantly grab-able if a customer wants double tumbler locks.
Drilling holes in the floor without authorization is a lease voider in many instances. So are adding more or different locks. I have seen some older apartments where every tenant just added a new lock instead of replacing a couple of older ones or having them rekeyed. You end up with a door that looks like it was from the old days of swiss cheesing race cars to make them lighter.
