I figured it out myself; it’s quite amusing. When the computer has no other option but to attack, it will stand on a Wall of Fire. If a weak unit, like a level 1 unit, is standing in front of the wall, it won’t attack because it decides it’s not worth it. This also works on magic-immune dragons, and it’s definitely a bug that has become a feature, which is used by creators of hardcore maps. I know I’m reinventing the wheel here; I just wanted to share it, in case someone doesn’t know.