How to decide what to do

[BlackwaterEddie]

That's the problem with having a high position (on top of a building or hill). You're going to have the best view of the battlefield but at the same time, the battlefield is going to have the best view of you

If I'm in a defensive positon it will usually fit the following criteria -

1. Easy to move out of in the event of surrounding area being overrun.
2. Hard to get to to ensure that I don't suddenly find a knife through the back of my head.
3. Good view of the area of interest without compromising myself.
4. Narrow arc of fire to enable me to put the majority of my shots in the smallest area, nothing like catching a squad in an alleyway when you're at the bottom with a SAW.
5. A position I can readily defend in the event that my position is compromised.

The majority of building interiors will give you most of this criteria and even though point 2 is hard to come by you can hinder any attacker by putting up claymores or other such devices in order to protect (or atleast warn) you from incoming attack.