The inside area is about 10'x8'.
The walls are 4' high with the center just over 6'.  So I can stand in the middle part and my daughter can
walk around in almost the whole treehouse.

The way up is through the trap door.  It's kinda a tight fit but it makes going
up and down a little easier.  You don't feel like you're about to fall!
 
 
 

Inside with the trap door closed you almost forget you're 15' off the ground.  I ran a 100' extension cord
out from the house.  It runs up the side of the tree.  We have a light and a fan.  All the windows are screened and it
really is bug proof.  The first night we slept in the treehouse no one got a bug bit.
 
 

The treehouse sleeps three with no problem.  It's a lot of fun when the wind
kicks up!  It has already survived one pretty good storm.  There was some
slight damage on the roof 4x4's.  I had only 6" nails holding on to the tree at this point
so I added four more 8" lag screws.