If I may chip in on this one shipmates.
When using my large fortresses I judge them from what I can glean about existing forts of that size. then I look at the sort of scenario I wish to portrait. It is no good having a fort that dominates your game so much that your objectives have no chance of success.This is where experience in the game is important.
The next step is to look at the forts which Ares print. You have a range from a very small battery of only strength 3 and firepower of 3. Sort of Frigate strength to a large edifice with 9 points of damage and fire power of 5. This could obviously take on a third Rate.
I then group cards together to give the strength of fort I want . Maybe enough to fight two First raters and a couple of Frigates too. This could be two strength 9 forts supporting each other across the bay and a strength 7 on an island further up the channel or group two cards to make one very large fort with two batteries, and a smaller one across the bay from them. There are also options of increasing the firepower of the guns. Some Algerine Forts or those in far Eastern parts had huge old guns that fired gigantic shot even some made from stone. Another way of beefing up your forts is to have one of them able to fire heated shot. Slow because of the danger of exploding the charges if too hurried and hard to carry from the furnace but deadly to a warship if embedded deeply in her timbers.
All things to consider when using forts against ships.
I will be discussing the use of plunging fire and extended range for Forts on headlands or cliffs on the optional rules forum shortly, so keep your eye on that too.
Hope this is of some help.
Rob.