Twelve towers fell from the heavens. Clear them in order, take their keys, and open the one tower still standing. It points home.
Twelve towers fell point-first from the heavens. The way through them is down.
You woke on the grass of Veyra with a mark on the back of your left hand and no way home. The mark opens one tower at a time, in an order you must follow.
Each tower drove itself into the earth when it fell. Its door sits at ground level and its floors hang below, five of them dealt fresh from the tower's own deck each time you step in, and a warden at the base holding a key. Die down there and the tower puts you out at the door with exactly what you walked in carrying. The tower keeps what it gave you.
Between dives there are towns. Work, rumors, shops, a bed. Everything you do up here makes the next floor down survivable.
Twelve towers. Twelve keys. One tower still standing, and it points home.
The Reverse Tower runs on the Gravebind engine: every floor is dealt from authored parts, every rule the Warden follows is a file, and the numbers are flat and honest. Every image sits on black, and the only light in the world is the arcane blue of the mark.