Until 1880, Veracruz was a walled city surrounded by mighty medieval defenses. The Baluarte de Santiago, the only surviving fort of nine, was built in 1526 beside what was then the waterfront. Slowly crumbling, it looks very out of place standing alone on the backstreets of the old town. It's not really worth paying the entry fee to go inside, despite the exhibit of pre-Hispanic gold jewelry that was discovered nearby. You can walk around the outside battlements for free.
It's also sometimes known as El Polvorín.