There are stone and wooden walls across the map. They are unbreakable, and normally form houses and other structures.
Wood walls only form houses, and seem to be comprised of multiple vertical logs. Stone buildings often don't have roofs, and are sometimes found scattered and ruined looking around the map.
Hitting walls will result in an effect similar to clashing: There will be a clang, a little flash, and the attack won't finish.
Doors are elements found in both wood and stone walls. Both start locked, and will open when unlocked.
Windows are only found in stone walls. They can be jumped through, and aren't found in enclosed houses. They can only be found in stone walls.
These elements are similar to windows, but the only way to get through is by rolling under them. They are not currently in-game.
Guillotine doors are also only found in stone walls.
Walls were introduced to wilds in the beta update when ruins were added. They used to be a more yellow, sandstone color.
When they were first added, these walls damaged players that attacked or sprinted into them. This feature was soon removed.