The shingles don't work well for a ceiling (shingles go on the TOP of a roof, not the bottom!) I would put a hole above the flag that you can drop down into, in case somebody's sitting in the corner of the flag room guarding the side entrances. It also gives you a reason to climb the walls. Some of the walls shouldn't have the ladder texture checked, though, like the doorways - there's nowhere to climb, and they just cause you to get stuck. As spook said, there's no real reason to go up high since it's so much faster to just go straight across the bottom.
I've uploaded the map to the beta server. I'll send you a PM on how to load it up so you can play test it with other people.