Lintel

you never place an arch. you make one inevitable.

a stone town where every block takes its shape from its eight neighbours: stack two and a window opens, leave a gap and a lintel drops across it

In Townscaper you never draw a building. You tap a spot and the game works out what belongs there from what is already around it, without being told. This is that rule, made small enough to see: every block redraws itself from its eight neighbours every frame, so putting one block on top of another cuts a window into the lower one and takes its roof away, and two stacks that end up beside each other with a single column of air between them get a stone lintel across the top and become an arch. Wall in a pocket of air and it becomes a courtyard, and something grows in it. You are not placing arches and courtyards; you are placing the conditions under which they are the only thing that can happen.

Concepts: emergence, cellular-automata, pattern.

An interactive you play with one thumb, on Wundr.