Age 7 to 18 Challenge Level:
The bricks must balance under their own gravity which is exactly modelled according to physical principles covered. The concept of a balance is explored throughout the school curriculum but is only given a formal mathematical description in A-level mechanics modules.
The final brick which may rest on the tower wall, although over reaching with this final brick may still cause the tower to collapse lower down!
For the hardest difficulty levels the tower is located 95% of the theorectical maximum distance the stack of bricks can extend without collapsing and without putting any pressure on the tower wall.
More advanced students may be interested in the fact that the wall can fracture at many different places, not necessarily just at the top or the bottom. Trying to spot the point of fracture prior to a poorly placed brick is not a straightforward challenge.