This is probably the biggest tourist attraction in Campeche.
The city walls were built in the 1600’s to combat ongoing issues with pirate attacks on the city. Initially there was eight bulwarks and 2.4km of walls, composed in a hexagonal formation. Currently there is only about 400m of walkable wall and seven bulwarks remaining (San Pedro, San Francisco, San Juan, Santa Rosa, Santiago, San Carlos, Nuestra Senora de la Soledad).
You can walk around the city walls for free. All of the bulwarks have information boards in english that can be read. There isn’t really any difficulty in knowing where to go given the obvious shape.
If you want to go up and walk on top of the walls the best place to do this is the Baluarte de San Francisco/Baluarte de San Juan which is located adjacent to the Puerto de Tierra (land gate) on the southern side of the hexagon. It costs 40 peso. With an entry ticket there is a few historical items you can view (mainly cannons) but then they let you go up and walk around 400m of wall and visit the above two bulwarks. This affords decent views over the central historical city.
If you do this be aware it is very hot in the middle of the day. Bring water, sunscreen and a hat.
Definitely worth doing while in mexico given the uniqueness of the attraction.