The Bluebook recommends the Dungeon Geomorphs, boxed with early printings of Holmes Basic, to DMs saying they “contain many suggestions and will prove very useful.”

ENCOUNTER KEY EXAMPLE

“5. False door which fires an arrow directly out when it is opened. The arrow is magical (+1), and if it fails to hit it will be usable. After the first magic arrow, it will fire only non-magic ones which will break whether or not they hit.”