or: Why is that fortress so damn important?
This is something I've mused over for many hours. The 7th Sea books can be amazingly detailed and they can also be hopelessly vague. The question here is this one: With two hundred miles of Rio del Delia, why is Montaigne so focused on capturing El Morro, and why does it serve as the lynchpin of the western Aldana defense? As presented, there are no answers. All it takes is a few moments looking at the map of Théah to realize the Montaigne should just go around the great fortress and either cut its causeways from behind, or simply bypass it and stretch their supply lines to capture San Cristobal where they would be able to, at once, begin reinforcing their army by sea and thus take Vaticine City without worrying about the forces at the Black Fortress.
Here, then, are a number of reasons why El Morro is the most important fortress in Castille and the Montaigne may not easily abandon its investment:
1) Attempting to cross El Rio del Delia would leave San Tropal exposed. The city of San Tropal, at least the way I have set things up, lies about a quarter or half mile from El Morro. If the Montaigne forces left in Castille (which are less than robust after the withdrawal of Montegu) were to attempt to forge another crossing, the garrison at El Morro would easily overrun San Tropal and its guardian fortress, thus pushing back the Montaigne conquest. Since a Castillian army in Torres would inspire insurrection and untold sorrows, the Montaigne cannot afford to take this risk.
2) The process of fording del Delia would take too long. By the time the Montaigne Armée Soleil had arrived at the location they were going to make the crossing and prepared to cross in force, a Castillian army would be ready to meet them. Well-supplied from its unbroken line to El Morro, this army would hold defensive territory while the Montaigne attempted to fight over a river, possibly in small rowing boats, large barges, or upon a hastily constructed pontoon bridge.
3) As long as El Morro stands, the river is in Castillian hands. Though Montaigne sea-power might be mighty, they have no riverine vessels capable of contesting the Castillian navy that regularly sails up the Delia. They cannot bring their navy to bear along the rivers due to their prohibitive width, thus meaning that whoever controls the most powerful fortress along del Delia's banks may also move ships with impunity. These Castillian naval vessels could easily sever contact from an army that crossed to the far side or, worse, destroy the army while it was crossing.
4) The garrison of El Morro can speedily redeploy. This is related to numbers 1 and 2 but is more applicable as a general rule. Once the investiture of El Morro ceases, its defenders are free to reposition anywhere they want to face the Montaigne army. As long as San Tropal continues making at least a showing of assaulting the fortress, they are trapped where they are.