Having lived in apartments for years before getting a place of my home one of the first and easiest modifications I would do was to replace the screws in the hinges and the strike plate for the door lock with stainless steel 4" long screws. I would also replace the strike plate with a reinforced one and add a deadbolt to the door if it did not already have one (pending landlord's OK). I learned to either keep the curtains closed or I'd put up venetian blinds and adjust the angle so I could see outside and get sunlight in while preventing passers by from viewing the contents of my apartment. Once I bought a house I removed many of the shrubs from the perimeter of the foundation to eliminate potential ambush hiding spots. If you have a garage entrance, once you pull in, close the garage door before getting out of your vehicle. Learn where the hiding spots are where you can see into your house and plant brambles or blackberries with thorns to discourage access to these parts of your property. One of the items I'd like to get is a protective film for windows which makes the glass very impact resistive. Great item if you have a window inside a porch. One can add home alarms and camera's either hardwired or wireless with battery back up but a better security system includes a somewhat nosey neighbor who can spot an unfamiliar vehicle and make note of it or call for back up. Somebody knocking on the door late at night ? Don't go to the door, check who's there from a window.