Rock

A Rock is a small (1 square) clutter that can easily block pathways, and sections off from the player, and monsters as well.

Tactics
Rocks can often get in the way of the player, and as a result the location of these rocks should be memorised when entering a room, so the player should avoid them and not get trapped on them and potentially attacked by high-speed monsters.

When Dungeon-making, rocks can be used in extreme abundance, making them great for interesting level design, with branching off pathways and maze-like rooms.