A cross key and its matching door

Doors and keys are among the most important parts of the Phantom Mansion games. There are four types of keys, and four types of doors to match them. Each represents a shape, a circle, a square, a triangle and a cross. To unlock a door, you must have the matching key, for example, a circle door is unlocked by a circle key. Doors and keys are first seen in level 1 of the Red Chamber, "Easy as 1 2 3".