Thin walls is a game design concept of having four walls inside every map tile. The idea was revolutionised by Michal Walczak, also known as magikmw, in his game Stabby. It is widely believed to be a feature of most future roguelikes, and it is rumoured that version 0.13.4 of Dungeon Crawl Stone Soup will be using thin walls.
Does not work well with out of the box line of sight and pathfinding algorithms.
Accusations of plagiarism
On August 2 2013 magikmw was confronted by roguelike developer vible who claimed that the thin walls idea had in fact been taken directly from one of his early roguelike works. magikmw has yet to respond, blaming Internet connection problems.