Gregory's ServerU.S. Forestry Service had these plans published for lookout towers over the years. Some are obsolete, sadly, due to better building practices, but I think there are still some good ideas in these
The footprint is 24ft x 21ft with a build surface on top about 15ft square. Total height is about 10ft, and although that's on the shorter end, it's actually doable for an individual
Would make a nice treehouse alternative
Plans for a 20ft tower. This is about as high as I'm comfortable building if I'm doing this by myself, or maybe with one extra pair of hands
I especially like the top deck layout of this in that it can actually support something like a tiny cabin