Campeche was founded in 1540 at the site of an ancient Mayan city named Kimpech. Its location on the Gulf of Mexico made it susceptible to pirate attacks, so a fort and defensive wall were constructed. Much of the original wall remains, and the city within the walls is listed as a World Heritage Site.
This is what the historic section of the city looks like from above. Notice the regular grid plan.
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We'll start off at the square you see in the upper part of the picture.