system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why were fortresses often built pentagonal?
gpt
It’s the fewest number of sides where the defense weapons of the time (cannon) could have overlapping fields of fire.  Cannons on both walls can target the same attackers.   

A 4-sided fort means each side is facing a completely different direction so they can’t do that.