15

There was a soft click inside the wall, and the heavy door swung inwards on its hinges.

Beyond lay a narrow passage, running behind the stone of the great hall. The air was cold and stale. Dust lay thick along the edges, but a strip down the centre had been recently disturbed.

Halfway along, a small round opening had been cut through the wall. Through it you could see the upper part of the hall, the top of the tree, and the empty place where the Evergreen Star had hung.

“From here,” Wesley said, “no one in the hall would have seen a hand at work at all.”

On the floor beneath the peephole a few needles lay flattened, as if someone had rested a ladder there, or leaned out awkwardly with both arms raised.

“At least one person still remembers these passages,” Snowell said. “Coalridge knows them better than any of us.”

At the far end of the corridor another door stood fast, locked from the other side. You rattled the handle, but it did not move.

Set into the wall beside it was a small brass plate with a row of worn pieces arranged inside a shallow frame. Snowell bent to look.
“It is one of the old safety locks,” he said. “A puzzle-door. The mechanism needs a very particular pattern to release. White cannot give mate here. The only way forward is to force Black to do it.”

This is a selfmate. White, moving first, must force an unwilling Black to deliver checkmate. Give the sequence of moves that forces Black to mate White on the fourth move (both White and Black makes 4 moves). If Black has more than one response, you only need to show one of them, and the continuation that still forces the mate on Black’s fourth move. Here's an analysis board, and here's Offerspill's Discord server, if you need help understanding the problem.