A Gothic cathedral is an argument in stone about where weight goes. Three devices — the pointed arch, the ribbed vault and the flying buttress — take the load off the wall, and once the wall is not holding the roof up it can be replaced with coloured glass. Build one. Push the height and the glass until it falls down, then work out which device you were missing.
The site1080–1300
Load a real building, then push it further than its masons dared.
The cross-section
The other machine the same towns built
The cathedral school standing next to your building site is about to incorporate itself. Open each one.