Kitty Tree

Cut the 2x2 shelf supports to your chosen footprint — about 21" gives a roughly 2 ft square. Put a pocket hole at each end, but offset each hole from the center of the support: because supports meet from all four sides, centered holes would drive screws into each other. Right-of-center on every one keeps them clear.

Cut the four uprights to your target height — the source went ~5-1/2 ft to reach a route off the stairs; match your ceiling and cat. Mark the level heights on each upright now, identically, so the platforms end up dead level. Drill pocket holes in the bottoms of the uprights to fasten down to the base platform.

This is the fiddly part, especially solo — build the frame up one side at a time, fastening supports between uprights at your marked heights. Work on a flat floor and check the frame stands square and doesn't rack before you commit every screw. Fasten the base first to give the uprights something to stand on.

Cut plywood platforms and drop them onto the shelf supports, letting them overhang the frame where a ramp will land — that overhang is the landing the cat turns around on. Cut ramps to span between levels. All ramps running the same direction keeps the tower tight against the wall, which is usually what you want.

Screw 1x2 offcuts across the ramps as grip cleats — steep ramps are hard on a cautious cat without them. Sand every edge, then wrap platforms and ramps in carpet or sisal for traction and scratching. Finally, anchor the top to a wall stud or wedge the tower floor-to-ceiling so it can't tip when the cat jumps off the top.