Neat letters had been painted above a narrow slot. “Ventilation Shaft Entrance: 2p.” Puzzled, Thunk reached into his pouch and extracted a tuppence. He dropped the coin into the slot, then drew his sword. Carefully, he touched the blade to the bars. The cylinder rotated. The bars swung down against the wall of the shaft. He shrugged, replaced his sword in its scabbard, and slipped through the open gate.

He left the lantern at the turnstile, braced his feet against one side of the shaft and his back against the other, and carefully and quietly worked his way down. His sword dangled from his belt, the point swinging gently. It was an easy descent, for he’d had plenty of practice at this sort of thing. Thunk had lost count of the number of impregnable fortresses he had penetrated by climbing through a ventilation shaft. True, Thunk would also be the first to admit that counting was not one of his strong points, but it was still a lot of shafts.

The opening above him grew smaller, the light from the lantern grew fainter, but presently Thunk could make out a dim glow beneath him. He had dropped nearly sixty feet and was well into the interior of the castle. A few feet later he reached the bottom of the shaft, which ran horizontally in four directions. The glow came from a square of glass set into the side of the shaft. Behind it was a candle. Below the glass was a small metal plaque. Thunk lay down in the shaft and put his nose nearly against it, barely able to make out the etching. It showed a vertical shaft descending against a black background and branching out into four horizontal shafts. At the intersection was a small dot, with an arrow pointing to it. The arrow was labeled “You Are Here.”

Thunk had plenty in the way of physical courage and a good deal of native cunning, but not much of a sense of humor. He grunted and unsheathed his sword, keeping it pointed in front of him. It was obvious now that he had descended into a trap. A trap set by someone who did have a sense of humor. Not a clever sense of humor, mind you, but some wise guy had made the attempt. Thunk looked at the entrances to the four shafts and debated which one to take. All of them, he suspected, would turn out badly. He considered climbing back up the shaft and forcing his way through the turnstile. Then he looked at the glass plate and the lamp.

Someone had to light the candle. Someone had to replace it when it burned down. There must be a door in back of it, one that led into the castle.