Not thick enough, apparently. They found two, and one was blowed up:

A second cannon, but damaged

Last year, Seymour found a second cannon at the same site. This one lay in an area that appears to have been where most of the fighting took place, with abundant projectiles and other artifacts.

This one was fired in the attack, Seymour said. The side of it had blown out, possibly due to faulty construction.

“What we're thinking is that they possibly did kind of a 'Hail Mary' type situation, overloaded it with powder and shot to try to keep from being overrun,” she said.

Best,
Ted