Note: This page is taken from n.wikia.com.
Doors are some of the most used objects in maps. There are three kinds of doors: one of them - the normal door - acts like a real door, which one can go through. The other two - locked and trap doors - act more like barriers.
By standing on the opposite side of a door, the ninja becomes protected from all enemies on the other side. On very isolated occasions, the ninja may be shifted from its present course by a door or stand atop the door's narrow width. This does not interfere with gameplay significantly and may even provide advantageous potential.