During the Undead Nightmare, El Presidio is a Safe Zone protected by high walls, multiple cannons and the Mexican Army. Ladders have been added to certain points on the walls for ingress and egress. There is a perpetual horde of zombies surrounding the fort. It is also one of two locations that the Missing Persons activity is linked.
It is a common location in Multiplayer to become a Public Enemy because of the many Cannons and Gatling Guns with one being present directly at the front door making it easy to rack up many thousands in bounty. Also, with having only two entrances it is a very defensible position and with a group of the player's friends (6 being the bare minimum with any number above that just to make it easier) they can man all the Cannons and Gatling Guns that protect those 2 entrances with people to spare for sniper duty (using Rolling Blocks, Carcano Rifles, Bolt Action Rifles, etc.) and guarding the inside of the fort if any Military break through the player's defensive line.
It is possible to see El Presidio from the top of Nekoti Rock, though it looks like an undetailed block. To do this, binoculars or a sniper scope are required.
In multiplayer, if a player takes control of a mounted weapon and fires at the walls of El Presidio, an additional $20 will be added to their bounty. This can be a very useful tactic for any players wanting to complete any challenges or achievements/trophies.
In Undead Nightmare, the Undead spawn in masses outside the front gate. The player can man one of the cannons near the gate and they can score the Mowing Them Down achievement/trophy by mowing them down three and four at a time. The other kill-based achievements (Long Arm of Marston, In Hail of Bullets and Bullseye) are also easily scored there, given enough ammo and patience.
This is the perfect location to obtain Undead Sharpshooter Rank 4, in which the player must kill all four Undead types in one Dead Eye meter. Conveniently, all four types continually respawn near the gates.
It seems that at least one person in El Presidio may become hostile towards John Marston.
In Multiplayer, shooting TNT or lanterns inside El Presidio will result in a $20 bounty. Throwing Dynamite or a Fire bottle will also result in a bounty if the explosion/fire touches any part of the fort.
Oddly, despite being a literal stronghold, it is not featured in a Stronghold match.
Underneath one of the sets of stairs in the fort there is a Jail Cell, though it is always locked, and an occupant is rarely seen.
In Undead Nightmare, Retchers can spawn inside of the fort and attack the survivors in it. It is possible for them to kill all but one survivor.
In Undead Nightmare a Retcher will rarely appear in the jail cells underneath the set of stairs or may accidentally spawn within the cell
In single player, wearing the Duster Coat while the Mexican Army controls El Presidio will automatically gain you a bounty.
Lassoing and hogtying a bounty target and taking him inside the fort will cause the soldiers to assume a combat stance. Cutting him free will cause the soldiers to shoot him until he either leaves or dies. There is no penalty for this, and the player can still claim the bounty.