Explain forts and castles to me

I’m not wholly ignorant of military affairs, but I must express puzzlement on a relatively basic matter. I often hear that “Castle X commanded the Y valley, and so Z and his army could not go that way.” Now, that makes sense to me if the valley is very narrow and the army cannot physically cross it without running up against the fort, or if the fort has artillery of some sort whose range can cover the valley. But this is often not the case; often in the cases being referred to, the area is miles wide, and the fort’s weaponry has a range far less than that.

To pick a concrete example, while in the Florida Keys recently, I was reading about how a fort was built in the Dry Tortugas. The original purpose of it was, I was told, “to protect the Gulf ports.” But this makes no sense to me. The passage between the Keys and Cuba is 90 miles wide; if I intend to take Mobile or Gavelston, I simply keep my fleet out of the range of the guns and proceed onward.

Please note that I’m not referring to the capacity of forts to act as strong points in a defensive line, as observation posts, or as the base of operations from which forces may sortie. Those I can understand. What I’m struggling with is the putative ability of forts to control an area larger than that which they can directly project power onto.

I hope that makes sense. And yes, I know what Patton said; my question is why anyone ever thought differently.

It occurs to me a lot of people won’t know the quote, so:

> …the capacity of forts to act as strong points in a defensive line, as observation posts, or as the base of operations from which forces may sortie…
But, that seems to be the answer to your question right there. “Fort/Castle Blank controls this blah blah” is meant simply as a metaphor for that.

Except that Fort Jefferson, the example I referred to, was none of those things.

Naturally, a castle or fort cannot defend any area.

However…if you are any enemy commander, moving your army through this hypothetical valley in the OP, you have 4 choices:

[li]Assault the castle, taking casualties from the hard-to-assault defences.[/li][li]Lay seige to the position, starving them out. Good, if you have a few months to blow.[/li][li]Try to take the castle by treachery/spies. Impossible, if the castle/fort’s commander keeps good discipline in the ranks, & knows you are coming.[/li][li]Bypass the fortress, & get your rear kicked by units attacking raiding your flanks.[/li][li]Or, go home.[/li][/ol]

OK. 5 choices.

Well, the fort represents a safe place for the your men to shoot at the enemy and retreat when when enemy shoots back. They can sleep there in relative comfort and safety while an army on the march has to haul their own tents or sleep on the ground, in varying and mostly unpleasant weather. Food can be stored for your men, while the enemy must hunt, forage or haul. It’s a major advantage in the pre- or early gunpowder era.

[QUOTE=Bosda Di’Chi of Tricor]
[li]Assault the castle, taking casualties from the hard-to-assault defences.[/li][li]Lay seige to the position, starving them out. Good, if you have a few months to blow.[/li][li]Try to take the castle by treachery/spies. Impossible, if the castle/fort’s commander keeps good discipline in the ranks, & knows you are coming.[/li][li]Bypass the fortress, & get your rear kicked by units attacking raiding your flanks.[/li][li]Or, go home.[/ol][/li][/QUOTE]
Well, that suggests that the real power of the fortification does lie in it’s capacity to sortie. If a fort was purely a defensive emplacement the butt-kicking in 4) would not occur.

For what, a day or two while my army goes around you? I’m not talking about attacking a fort, I’m talking about avoiding it – going around it and proceeding away from it.

This makes the most sense to me; the logistics involved in provisioning and the threat to supply lines.
But neither of these put me any closer to comprehending the purpose of *purely defensive * emplacements like the one in the OP.

The forces inside the fortress do much more than make sorties. There should be a sufficient garrison to intercept and bring to battle any enemy army that passes through the fort’s intended area of control. Remember, pre-gunpowder fortresses had artillery with less than 500 meter effective range. Almost all their power was in their garrison.

Make sure you factor other home-advantage issues into this…the attackers have little or no idea of the terrain, whereas the defenders know everything from river-crossings to sources of food.

But the ‘attack from the rear’ is something that would be devastating for such an army. Your fighters are at the front, and there is no way to get them back that is faster than a horse. By that time, your support system could have been wiped out, after which you’re not a strong army, yet still deep in enemy territory.

I agree with all the points that have been made. Having a secure base of operations with guaranteed food, water and rest is a big advantage.

Remember we are talking about times when you had to lug all your own supplies +weapons around, perhaps with a few horses to help.
Suppose the main road runs beneath the guns. Are you going to slog through rough country pulling cannon?

I couldn’t find much on your specific example, but it is described as a ‘coastal’ fortress. So presumably all enemy fleets have to go the long way round (which weather conditions may make difficult). Plus any defending ships can shelter under the batteries of the fort (and resupply / repair there).

Well, if guys in the castle never leave it, fine. More likely, the encamped army will be sending out scouting patrols to look for enemy trails. If the force you’re sending is sizable, you certainly won’t be able to hide it from people intimately familiar with the area, and then you’re subject to sneak attacks, having your supply lines cut, etc. by guys who then hastily retreat to the safety of the castle.

But that’s precisely the point. If you bypass the fort, the area around it is under ennemy control. It can attack you from behind, it will cut your supply lines, can block your reinforcements, and if ever you have to retreat you’ll be attacked by the garrison you left behind. You can’t safely leave behind you ennemy forces than can operate in your back as they see fit.
Ignoring the forts basically amounts to being cut off from your rear lines/own territory. For instance, if you need convoy of food, supplies, etc… you’ll need to send with it enough troops to fight the fort garrison. Maybe you’ll have to divert less troops than you would need to besiege/attack the fort, but on a permanent or recurrent basis.

And of coure, fortifications denies you the use of facilities they could directly protect (food, a harbor for your ships, money, train stations,…) and secure them for the ennemy.

Not really. The passage between Cuba and Florida is 90 miles wide … I wouldn’t think keeping five miles away from the Tortugas would be that hard. But I guess "sheltering under the batteries is the idea … although it wasn’t a really a naval base; more than one large ship couldn’t dock there.

Well, actually many of the river forts and valley forts / castles were built in a time when most often waterways and river valleys were the main passage between areas. People tend to travel on routes that are passable with relative ease [hence you go through mountain passes instead of up and over the top… or along a river on the nice pretty flat land that also happens to have a convenient road on it instead of hacking through the wooded hillsides]

Really, europe was not anywhere near as densly populated as it is now until the 1600s and 1700s…the black death literally decimated europe in the 1300s. Actually, IIRC in Barbera Tuchman’s Looking Through a Distant Mirror, something like 30% of the population of europe was killed. That is a serious depopulation. The amount of cleared land was very contingent upon the lay of the land, you clear nice flat farmland, then you do rough clearing of sloped land for grazing, and you reserve steep wooded land for a fuel and building wood source.

You certainly dont put roads where you are not needing them, so being able to do an end run around an established fort/castle isn’t something taken lightly. Armies on the move had not just the infantry marching, teh horsemen and mounts, but could also include field weapons ranging from trebouchets to cannons depending on the year. You had carts holding the basic weapons, ammunition, tents, ‘furniture’ [blankets and bedding, all the way up to and including tables and chairs. IIRC Caesar actually traveled with a mosaic floor in sections] In addition to the army itself, you also had a mass of camp followers ranging from soldiers wives, assorted laundrywomen, cooking women and even prostitutes/kept women. I would like to see an average army and baggage train go cross country in our equivalent of the Donner Pass and not take the pass, or travel the equivalent of the length of the appalachian trail without actually using the trail.

In conventional war we do the same thing still today. The so-called from “line” is not a line of troops standing shoulder to shoulder. It is a series of mutually supporting strong points (read forts) with the major forces concentrated at places that are either easy to defend or well placed for attack.

Since today’s war is a war of mobility the forts are merely temporary concentrations of forces, but they serve the same purpose as old time forts. A place to keep armed forces in relative comfort, with supplies at hand. The forces are ready to move in either attack or defense on short notice.

To expand on what others have said: never underestimate the importance of a safe haven. Your enemy forces have nowhere safe to be in that passage. They have to survive without resupply, without repairs, with never a moment’s peace. You can hit at them, and if you suffer damage you pull back to the fort so you are under its guns, and stay there till you’ve fixed your damage or resupplied or whatever.

You don’t need to dock, you just need to anchor within range of protecting guns and small vessels for resupply.

One thing to keep in mind is the technology of the day.

If we are talking the Age of Sail, the benefits of the fort increase. Presumably, there is a high spot of land in or near the fort which can be used to see a long way and to signal to the ships in the area.

So, one local vessel sees enemy/suspicious ships making the passage through the strait. (With tacking and relying on good wind to move, they probably wouldn’t hug the Cuba coast very closely.) The local vessel sails to within sight range of the fort (land elevation and ship mast height make that a good distance), and signals the presences of the enemy ships.

The fort begins signalling other local ships to coordinate a response based on the expected opposition. With most winds, they can modify their tactics to be upwind, or at least level on wind with the opposition (a huge tactical edge).

So, now, any fleet attempting to sail through the channel is facing a coordinated response from a fleet with knowledge of local wind and current conditions, with presumably well rested crews and fully repaired ships. That’s not to say the incoming fleet couldn’t prevail, but it is a risky venture. So, nobody is going to try it on a whim.
In the Age of Steam, the benefit of wind direction disappears, but the benefit of the local fleet being able to use coal more liberally gives them the same edge.

A single ship, or a small number of ships, might be able to make the run without interference but the fort does a good job of protecting against large forces.
Summary: The main benefits of the fort are 1) safe refuge for fleet, 2) command and control and 3) supply and repair for defense fleet.

A fort is also available to ensure that sovereignty is maintained even when there is no enemy attack.

It the maintenance of sovereignty that is important, you can levy taxes, you can conscript and you can govern a province from fortified headquarters.

A great many fortifications are less to deter enemy attack, and more a statement of authority to the local populace.

Those that are placed around headlands are strategically placed to deny safe landing points, especially in the days of sailing ships where unfavourable tides and shallow waters severly limit the ability of an enemy fleet to land and resupply.

Taking on land based fortifications in a ship is risky at best, the ship cannot take anything like the damage land forts can, and a ship would certainly not have access to extra resources for resupply and repair.

A fortress or fortification is tactically defensive but strategically offensive (usually). In short, it’s meant to project military power further with a solidly defensible base. This allows you to engage large numbers of the enemy with miminal losses and available forces.

Other methods include using forts as laast-ditch defensive points. While better than nothing, you obviously don’t want to have to use them. If things get that bad, victory either way is a huge toss-up. ROme was particularly good and using forts to defend and break other’s.