Terrain
There are various types of terrain in the game, many of which don’t do anything.
There are several decorative variants of the basic floor and wall tiles. (Some of these may well acquire gameplay significance, particularly bone walls/floors.)
Pools of various liquids can be found in the dungeon. Water is just a decoration, currently used mainly to represent the baths in flesh-demon shrines. Lava and acid are harmful. Lava does 2d10 damage when entered. Acid does 1d10 damage and damages the player character’s armour. These effects are negated if the player has resistance to the relevant damage type.
Doors obstruct line of sight (unless the square is occupied) but not movement, and can be detonated with a wand of shattering.
Altars, anvils and tombstones obstruct neither line of sight nor movement. Anvils can be detonated with a wand of shattering. Tombstones probably should be shatterable, but aren’t as of 395a18cf5455aa0c407be706255c1f6623877885 in master. Altars aren’t shatterable and never will be. (The demon-gods of the dungeon are powerful therein.)
Furnaces obstruct movement, but not line of sight, and can be detonated with a wand of shattering.