The general assembly was no problem with two people. Instructions were clear, and easy to follow. 
Four stars because:
A few pieces weren't pre-drilled for dowels and hardware. I had to drill them myself which was fine because I had the tools to do so, but would have been a real pain if not.
There is a lot of exposed hardware, mostly the heads of the bolts and some angled fasteners for the stairs which detracts slightly from the finished look.

But the kids love it so there you go.