Floor Level: Pick a moderately high floor – 3rd or 4th floors provide protection against thrown objects from the street.Highest floors (15th or higher) may be problematic in a fire alarm evacuation.There are a bunch more. I would add that if someone knocks on your door, you should also call down to the front desk. If the person is not security (or the police), then the front desk guy will call security (or the police).
But you did bring your .45, right? ;-)
This actually happened once to Mom & Dad (who were tragically .45-less at the time). But they called down, and soon the local Po-Po were doing a security sweep of the building.
RTWT, which has excellent suggestions.