Take a look at how valve designs rooms, and try to make some observations as to why said interiors work so well. Is a corner hallway a challenging, but fun way to push? Are there alternate flank routes available for spies and scouts to ambush the defending team? Is there a one way door that cuts off a flank, forcing players to take a new route? I could go on, but those are the sort of questions you want to ask yourself when implementing something into your map. Just try to identify what makes good maps fun, and replicate it into your map. Hope this helps!