I’m pretty sure the treehouse was one of the monumental structures of the show, considering the details put into it. This is a concept sketch of Twilight’s Treehouse by Dave Dunnet. 

This sketch was located from the MLP Wikia under the Lauren Faust gallery. But searching around got me Dave’s name. You’d have seen his work on other collaborations with Craig Mccracken and Lauren Faust, as backgrounds and locations on The Powerpuff Girls and Foster’s.