FillField is a quite difficult puzzle game with extremely simple rules.
The complexity of this puzzle is determined not by the complexity of the rules, but by the complexity of the task. Rules are trivial: there is a rectangular box filled with dual-position switches. When you change the state of one switch, this leads to a change in the status of some (what exactly -- depends on the settings of the game) nearby switches. The goal of the game is to set all switches on the box in the same position.
P.S. Interface of this game is really TUI (https://en.wikipedia.org/wiki/Text-based_user_interface) but not strictly ASCII: I use Unicode because of some letters like arrows and such.