Step 4: Tools
The bare minimum of hand tools: hammer, saw, level, square, tape measure, adjustable wrench. Power tools: cordless drill, jigsaw
Useful but not critical power tools: miter saw (cutting lumber to length), table saw (ripping lumber), router (rounding edges).
A ladder is important, but even a stepladder would be OK if you installed a ladder/rope ladder/stairs to the treehouse early in the build.