From The Mana World
An Action layers is a tilelayer that is build in with every map using Tiled. They are mainly used to add multiple layers on which you can walk. Some examples are: a city wall, a bridge or any cliff-like situation.
These layers, just like all other map layers, consist of tiles. There would be at least 3 different tiles required for these layers: a walkable 'check' tile, collition tiles and a transition tile. The 'check' tiles could be seperated in spawn tiles and non-spawn tiles to define areas where monsters should and shouldn't be spawned (like in narrow passages and near portals).
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