Thanks for this thread, and thanks for this map. (I have to admit that I have ever checked out the scene editing area)
I have had a lot of fun playing on this map, and in essence it plays a lot like one of the old siege maps on the community server (I can't remember its name, but the tile set was yellow bricks and there was only one set of stairs to the top of the keep). At first glance this map looks heavily slanted towards the defenders, but once attackers get a foot hold on top of the keep and cut off reinforcements from using the stairs, the map turns out to be only slightly advantageous for defense. I think that the suggestion of moving the attackers spawns forwards is really all that is needed to even the playing field (that or lowering the entire keep structure, although that might be to much work).
Now for the things that I think are great about this map. First of all the flag area makes for a great fighting ground, the different levels, the sectioning of the area, and barriers (this area alone turns flag camping from a boring necessity into a fun task). I love the side rout you can take after climbing ¾ of the back door stairs (which involves some risky jumping) because it allow a couple of guys to sneak onto the flag area, while making it impossible to take a large force through this route. Lastly, this map looks spectacular; kudos on it's scenic quality.