It turns out that the fort had some structural irregularities. By the time Peach went down for her nap today, the short wall had leaned over so far that it was beyond repair, while the longer wall was also listing dangerously in the middle. I cannibalized the short wall to buttress the longer wall:
and then built a square tower where the short wall used to be.
At that point, Link was forced to concede (when pressed) that I am the coolest Mom on the block. I plan to treasure the memory of that statement when he is a surly teenager who is embarrassed to be seen in public with me.