it has 2 issues...First, the "round" sides and back walls are made up of interlocking segments that fit together sort of like links in a bike chain. That whole systems is sort of tricky to get into the exact right place. And since it's all connected, when you fiddle with one part to make it fit just right, it can cause another part to get out of place. The other thing is the way they top skin comes together. They're wedges that are hinged along the perimeter. It's a little tricky to get them all to lay just right, particularly the ones on either side of the cockpit. The whole thing just seems to have tighter tolerances than any other lego kit we built. And since it's so big, it's hard to work on one part without disturbing another part. We got ours together about 99% complete one time, but it now sits sub assembled in a few pieces.