When you visit the Mexico City Metropolitan Cathedral in the Zócalo, know it is an active church with free entry most of the time but expect security bag checks and occasional closures for services or events. Go early to beat crowds, pickpockets and tour groups, and leave large backpacks at your hotel; if you want skyline photos, ask about access to the bell towers because they open in limited numbers and sometimes charge a small fee. Dress modestly if you plan to attend mass, be respectful during religious ceremonies, and combine your visit with the nearby Templo Mayor and Palacio Nacional to make the most of the area.