I know that if you are able to save the soldier gained in Santa Domingo from being sacrificed then send her to help defend the fort there, the garrison commander and Umbarto survive and may show up later on the Spanish Main but, as I am unlikely to not save her in any future games, I'm just wondering, does anyone have to be assigned to run the garrison of the fortress if they do not survive the coup? <spoiler alert> I'm thinking the fortress is just a money sink - seems to have no real purpose except to keep the story line going. Does the repair options actually do any thing? It seems that the battle, 18 to six, is unwinable (and where the heck is Noriega and his finely trained company of soldiers and why were they even mentioned?) and doesn't really affect anything as far as I can tell.

Do you mean the Fortress in Mexico? The one containing a church for morale boosts, a pub for morale boosts, a workshop for free crafting materials of your choice regularly, a trade centre which is always stocked well and you can get decent prices at, not to mention the stables that not only replenish your moves but double them, and the most important one - the storage area where you can stash your gold so you don't lose many thousands in one bad camp event - that fortress?

Surely not, since the stables and storage alone make this more than worthwhile.

I don't think you have to get one of your troops to run it though if Noriega's not there - not including when it's first refurbished of course. If Noriega isn't there, there MIGHT be an option to send one of your officers to handle half of the soldiers in his stead, but I can't recall 100% if that's actually the case. If not, then without Noriega the fortress (as a source of troops and to help in that battle) would indeed be close to useless (if we ignore all the other benefits and just focus on that particular battle) - which is why I'm inclined to think that as long as you have enough troops in the fortress when the battle occurs, that you'd have some option to get a similar outcome as you would if Noriega were there.

Noriega is mentioned because he takes charge of "half" of your troops - the troops that arrive in your fortress over time once you finish building the barracks. Without those troops (or with not enough troops, or no one to lead them) - that battle is MUCH harder. It's definitely winnable otherwise, though it's most certainly a challenge on the hardest level.

It's much easier with higher tactics, provided you have the cannon and enough traps and barricades (since your tactics score affects how many of each trap you can place, as well as how many cannon balls and/or oil lamps you can lob). That said the only thing "easy" about it is that it's still easy to screw up and lose.

Personally I use barricades and caltrops to slow the enemy down as I pelt them at range, and make extensive use of soldiers to use their protective abilities to cover everyone else (and to deal big damage once the enemy gets in range). A few cannon blasts don't hurt either, or use oil lamps if you don't have the cannon. Smoke can be useful too so you don't get peppered with arrows (as much).

I have no idea if losing affects any future battles, but I'm pretty sure it would reduce your troop count at the fortress at the least - which means you'll likely have one option unavailable to you near the end.

I'm sure there are many other ways to beat that fight, depending on what stuff you have available - some of the extra-secret bonus objects can likely be very useful, though IMO all but one feel a bit OP so I try not to use them (or even if I don't consciously try not to use them, I tend to forget I even have them).