*sigh* Newbies.
99% of all apartment break-ins are executed via a window. But, there are different circumstances.
If you live in a REAL apartment, i.e. a multi-story building ( I grew up in one), then most break-ins are executed via the fire escape. Those windows that lead to the fire escape need to have GATES across them. NOT locked with a padlock or key, but a simple lever. Trust me; there are gates made that release with a lever, but are IMPOSSIBLE, even for a double jointed person to release from the other side.
If you live in a "private house apartment" like where I live now (two story, eight units, four units per floor..think "clover shape") Then most break-ins are executed via the sliding glass patio door that leads to your patio or balcony.
This door needs to be secured THREE ways:
1. Lock the door so it can't simply be pulled open
2. Prevent the door from being LIFTED off the tracks by drilling holes thru the frame AND the door at the top and inserting long nails thru the holes.
3. Prevent the door from being pried open by putting a piece of broomstick or closet rod in the track on the floor. Long enough to basically brace the SLIDING door against the frame against the wall on the opposite side.
The above three steps WILL NOT prevent them from simply smashing your window and climbing thru, but noise is something a thief wants no part of.
An active alarm (monitored) w/motion sensors is your second line of defense in case they actually get IN the apartment.
RENTER'S INSURANCE you should already have. Period.
Hope this helps.