The Black family must have tremendous military and political power. Even accounting for game play and story segregation for building a civ at every single level, based on the enemies they are fighting (Circle of Ossus is well armed enough to occupy Havana and the Blacks are confident enough to launch an attack on an outpost of the Ottoman Empire away from their homeland) as well as the fact they have minimal oversight and orders from higher up suggests that the Blacks are an army onto themselves. A small scale version of an N.G.O superpower perhaps?
On a similar note: fishing boats can gather food from fish or coin from sperm and humpback whales. While gathering food from fishing site eventually exhausts the food from it, a whale as coin source is infinite! That's right: throughout a game, you can station a maximum of four fishing boats around a whale (and there is usually more than a single whale on the map) and have them gather coin uninterruptedly until you move them away from the coin source or they get destroyed. Needless to say, even though at first we thought we'd never run out of them, whales turned out to be a very finite source of coin as time went by.
Morgan Black is over 30 at the time of the Siege of Malta in 1565. John Black, his grandson, is in his 20s when the Seven Years' War breaks out in 1756. This looks like a pretty evident case of Writers Cannot Do Math until the end of the campaign, when it is revealed that while the Fountain of Youth was destroyed at the end of the first act, its water was still available and that Morgan and probably Elizabeth drank from it. Morgan is in fact still alive by the freaking 1840s, although as a very old man..
And it's also possible that the Fountain of Youth slows aging.
Aztec's pyramids double as fortresses in both games. At first it felt silly but then you realize the Mesoamerican civilization's pyramids are not simple step pyramids but the average joe will have difficulty climing up the stairs. Add in archers barraging you with arrows and it becomes an impregnable defense.