Action Code Script (ACS) is the scripting language that was created for Hexen and has been greatly expanded by ZDoom. ACS enables level makers to script events during gameplay, making the creation of interactive environments even in Doom's archaic engine infinitely more open-ended. With very basic commands, an author can modify the structure of a level in ways such as raising and lowering floors separately, simultaneously, in the same or opposite directions, and to any height or depth. One can even use move walls, given that they meet certain criteria (see Polyobject). Textures displayed on floors and walls can be changed. Monsters — and any other actors for that matter — can be inserted, removed, monitored, have many of their properties altered, and even pursue "objectives" through a limi