One idea.
Write down the numbers 1-60 on a piece of paper. Starting after the first turn, each turn you cross the lowest number off the list.
Each turn you roll 1d100. If you roll any of the crossed off numbers that's the same as a tower collapsing.
This gives you a 1% chance of disaster after the first turn going up 10% after 10 turns, 20% after 20 turns, 30% after 30 turns, 40% after 40 turns etc.