Consider a simplified version of Lode Runner without enemies or gold, and in which the player can choose when blocks are solidified. We provide a formal Coq proof of the answer to this question, namely:
- There is a rectangular stage which allows the player to move from top-left to bottom-right and top-right to bottom-left, but not vice versa.
- Such a stage must include ladders and bricks, and those two tiles are also sufficient.