Here is probably more info then you want on the treehouse. Look at the pictures and then read this.
Professional carpenters built the treehouse, which we refer to as Halina's Triahouse in honor of two of my daughters Halina and Tria. It is a stud wood frame structure. It is insulated and has a metal roof, which has a 20 year fade guarantee. This treehouse is built to last.
The tree is a big leaf maple, and is very much a live tree. All exposed beams are treated 4x6 and are attached to the tree trunks with 3/4" stainless steel threaded bolts with washers, lock washers and nuts. The lower collar tie beams wrap the four trunks so that together with the platform of the treehouse the trunks act as one unit of support. The 4x8 and 4x10 support beams for the platform are through bolted to the tree as well. Anywhere that a beam is attached to the tree the bark and outer cambium layer were removed and the scar was first painted with a tree sealant. This is so as the tree continues to grow and expand the trunk will grow around the beams and not keep adding rings to push on the beams and create forces which could tear the supports apart.
Although there is not a good photo of the access included, it is important to note that it is up about 8 - 2x4 rungs of a ladder and through a hole in a platform approximately 4 feet below the main entrance to the treehouse. There one removes their shoes. Then, offset a couple of feet, is the trapdoor entrance (about 18" x 24") in the floor of the treehouse. Although it is almost impossible that even a small child could fall through the entrance in the floor, the structure was built in such a way that should that occur they would be caught by the platform below.
There is a Dutch door on the backside of the treehouse, which opens from the waist up. Right outside that door and slightly above is a shaft and 10" pulley mounted on a structure similar to what one might see in a barn for raising hay up into the hayloft. This rope and pulley system is for raising toys up to the main level. The purpose of this system is both safety and convenience.
2x6 floor joists support the floor, with plywood sub floor over the 2x6's. The place is overbuilt if anything. The finish is immaculate; the clear cedar interior finished like a fine cabinet.
The electric toilet is all stainless steel construction coast guard approved for zero tolerance installation. The electrical outlets are gfi protected. The enclosure is weather tight.
The view looks west off of Vashon Island, out over the West side passage and on to the Olympics centering on The Brothers, a unique set of peaks.