The Mountain Walls were walls made by Tytanowy Janusz, one of the two builders so far. They were added in the game in an earlier update that also included a new Ruins map. The mountains replaced the old map, which featured indoor play and bigger markets. The Mountain Walls are currently both in Ruins and CTF (Capture the Flag). It is impossible to climb up the walls, unless you have a Spear, and use its special. Falling off or jumping into or being flung backwards from knock back into a mountain wall causes 0.5 damage to the player.
This is an example of what the community can do to help the developer, Rezoner, improve the game. Here are some pictures of Tytanowy's mountain walls: