Build a puzzle. Everyone else has to walk it.

The form is the editor. Raise walls by clicking, then walk your own route from 🚩 to 🏁 — that walk is the answer, and it is recorded as you go. Nothing is typed: publishing sends the walls, the start, the goal and the route to /f/demo as ordinary fields. When a visitor walks it back, the server replays their steps against the stored grid and decides whether they got there.