A cage. Not like a trapping cage or anything but something that could easily hold 5-10 average sized people.
I was hiking in the mountains just above the house i lived in, at the time, in Missoula, Mt. Took maybe 45 minutes of walking, mostly uphill and without a path, to get to the spot. Round steel bars for the edges and rope instead of chain link for the walls and ceiling. It was all set up just on the far side of the ridgeline i was walking, so it couldn’t be seen I suppose.
Nothing around it, no foot prints or tire tracks or anything. The rope looked undamaged as well. No idea what it was for but it definitely creeped me the fuck out.