The castle has a single moving part, just the door at the bottom. While he waited for Mike to help him with it, he put together the Troll tower cart, which has a multitude of moving parts like a battering ram and a gang plank that pulls out, and a very complicated set of instructions.
How I wish I could see it from Nathan's perspective... that the simple one in my eyes would be so much more difficult than the multi-step project where the success of one moving part depends on the preceding five steps being done accurately.
He's a smart kid though... I suspect that he really thought that the simple one was "boring" and wanted to do the cool one by himself and leave the grunt work to his minions.