Wikisimpsons - The Simpsons Wiki
Bart often comes to the treehouse with friends, mainly Milhouse.
It once housed a casino. 
When Bart and his friends were trying to listen in on Lisa's tea party with her friends, Lisa saw the paper cup which the boys were listening from, and pulled it. Then the boys started pulling it, and it turned into a tug of war, until Bart's treehouse fell apart due to the pressure of Lisa's pulling on the tree. Then, after Homer's disastrous attempt and getting a plan for the treehouse (which turned out to be something created by a person Homer doesn't even know) and chopping down trees in the forest for wood, Marge called on the Amish. The Amish put together the treehouse pretty quickly, but the grand opening turned into a disaster when Homer switched the electricity on (put together by the Amish) and the electric wire exploded and the whole treehouse was set alight. Homer got trapped in the treehouse, but when Santa's Little Helper went to "save" him, he dragged out the turkey instead. Snowball II ran up, and Homer, unconscious on the floorboards, was woken up (or rather scratched on the chest) by the cat, and was lead out of the window safely.
It was destroyed a second time, before being rebuilt, along with the Simpsons' house.
It is a small house made of wood with a ladder leading up to it and a secret entrance in the floor. It's got a couple of windows as well as the door. The interior of the treehouse often changes, usually it contains only rudimentary furniture, but other times extravagant furnishings.