It is a high-level FFI binding with a lisper-friendly interface. It can be obtained from Github here: https://github.com/Lovesan/doors.
It uses Virgil as a back-end. It also depends on trivial-features, alexandria, trivial-garbage, closer-mop, and bordeaux-threads.
Doors is also the name of an "embryonic" attempt at a window manager for McCLIM: https://github.com/admich/Doors.