Continuing the architectural theme, Isabel at God Plays Dice has a post on the ultimate fate of the real world Königsberg bridge problem. Königsberg had seven bridges, and in 1736 Euler proved it was impossible to find a path that allowed you to cross each bridge exactly once.
In World War II, several of the bridges were bombed, and later some were replaced. In present-day Königsberg, now Kaliningrad, there are now only five bridges, and you can now find a path that allows you to cross each bridge exactly once.
Fascinating! My father perhaps had inadvertently contributed to the solution of the problem - in April 1945 he swam across Pregel pushing in front of him a radio transmitter precariously perched on an empty wooden box — to direct shell fire of his artillery battery. The use of bridges was out of question.