A little carpet adhesive, a few staples, a handful of small brackets, and it's almost complete! Overall, it's solid as a rock and has enough mass to keep it stable without anchoring it to any walls.
Only things left are a ramp from ground level to the second story across the front and potentially a hammock somewhere(I'm struggling to see where i could put one without compromising some other useful part of the tree).
Thoughts are welcome!
Untreated 4x4, which I think cost like......$10-$15 maybe? Then carpet adhesive was like $20, some wood screws, and more staples. I've probably got $60-$90 in it, but it would be pricier if I didn't have the brackets already.
It's hard to say exactly, but I will say that if you bought everything you needed from, say, home depot, it would be very expensive, probably 2-3× a similarly sized store bought model. But the store bought ones are small for large cats, so your mileage may vary