Cair Andros

So Cair Andros is essentially a fortified river crossing. Due to the fact that Osgiliath is a city, closer to Minas Tirith and also a major river crossing, it naturally would have priority in terms of troops. But this would not be a major problem if handled properly. Solid fortifications can turn the terrain here into a massive defensive advantage, which in turn allows for fewer troops to be good enough. Before we get into this though, we need to understand exactly what role this fort would be playing.   There are going to be three main types of traffic passing though. The first would be boats going up and down the river. The other would be civilian travelers and traders. Lastly, there would be armies - most likely hostile Easterlings and orcs. Which means that Cair Andros would on a day to day basis be geared towards inspecting cargo and probably patrolling the river and roads in the area to prevent piracy and highway robbers. Part of this would involve having to stop ships and ground traffic and the collecting of tolls. The area would probably also have lodging and supplies for travelers too. So for day to day operations, Cair Andros would 1) facilitate legal trade 2) identify and stop black market operations 3) law enforcement and 4) tax collection.   But we have to admit that this is not where the most difficult threats would come from. Invading armies would fill this role. Which means much more heavy fortifications than one would need for dealing with traders and run of the mill criminals. So it would need to be able to resist sieges like a normal castle would. Part of this would mean the ability to scale up the troops garrisoned here. Now, the Anduin is a normal river. If just too wide to cross normally at other locations. Which means that Cair Andros would be the easiest crossing and thus a natural target. But the idea of pontoon bridges and smaller boats are not new ideas. Which means that Cair Andros would be Plan A, but is not essential for an invasion. Should it prove too much of a problem, attackers will develop alternatives.   So from a military point of view, Cair Andros needs to withstand the initial attack so that invasions would be stopped. But it also needs to be set up to strike out to raid the enemy's alternative crossing points. This means that it would be a likely base for cavalry and river boats in the remnants of Gondor's navy. These raids sound perfect for Rangers.

Now, the river itself would be the biggest defensive advantage for the garrison and would probably serve as the main line of resistance. Basically an in built moat. So those same bridges that were set up for civilian traffic would need some form of flanking towers on the ends. On the land side, these towers would ideally be the only defenses, functioning militarily mainly as watch towers and protection against surprise attacks. Essentially, their goal would be to stop the enemy from taking the bridges long enough for the guards to retreat to the island and pull up the drawbridges behind them. Essentially, a gate house and palisade along the river the length of the island would all that would be needed on the land side. The stone gatehouse and flanking towers would need to be pretty strong, as they wound be the first thing attacked. They would also be the first point of contact for civilian traffic. So these gatehouses would serve as the first inspection and toll collection points.   Now, why did I put in the palisade? Well, as the defenders would hide behind river itself, major defenses on the land side would not be needed. Nor would these provide any extra law and order benefits. But it seems highly unlikely that the entire length of the island can have walls built right up to and touching the water. There would have to be some areas where the walls would have to be off set enough for a boat load of troops to land. Which means that the land side palisade could be useful in keeping a landing force from launching against these (relatively) weaker sections of the island's main defenses.   The bridges themselves need to be a combination of practicality and defense. One would not want to constrict the traffic any more than one has to. This means wider bridges across the river so more wagons and foot traffic can get across. To prevent river traffic from being impeded, I think the bridges should be taller so boats can go underneath. For similar purposes, I think the towers should not intrude too far into the river. But at the same time, one wound need to make sure that the enemy armies would not have too easy of a time. One tactic would be the drawbridges I mention later. The other is an expansion of the Zig-zag bridge design used normally as pedestrian bridges in Japanese zen gardens. These turns would make it harder for troops - especially cavalry - from maintaining their top speed for long. Which then means they will be under fire from the island defenders longer.   Now, a note on draw bridges. I suspect that the weight of traffic across the bridges would need sturdier, permanent construction. But it is entirely possible that sections right next to the gate houses on either end of the bridges can be raised and lowered. One does not need to remove the entire length to make it impassible. Using sections at both ends of both bridges would have important benefits though. No army is going to attack from both sides. If it was able to do that, the attack would be pointless as it would already be able to cross the river. This means that one land side gate house could pull up their draw bridge as the first line of defense and the other as the extreme last ditch effort. The ones on the island serve the same basic idea. The attackers would have to cross two draw bridges to get to the island and two to get off, adding to the difficulty of the attack with little extra cost.   On the island itself would be the main walls. As would be the docks for the boats needed for the raids. Now, I suspect that there would be one primary wall instead of many layers. This is based on the idea that the river would provide a lot of protection, so layers would not be needed. And all though the island is pretty long, it is not that wide. Concentric rings of defensive walls might not leave enough space left for the essential buildings.   But normal castle design includes a keep that could be its own reinforced strong point and fall back point. Due to the fact that there are going to be both civilian and military areas, I think that there would be defensive walls forming internal divisions. Which then adds layers of defense without distinct rings. Lastly, each tower can have fortified doors, store rooms, minor workshops and sleeping areas if they are big enough. This then allows each tower to hold out after the rest of the fort has fallen. The combination of these means that there is still layers of defense that would make an assault bloody and difficult work. Even if the outcome is inevitable and there is not any concentric circles of walls.   For the civilian law enforcement and trade purposes, a smaller garrison would be needed. But for an important river crossing one would need to scale that up in times of crisis. But how many? As I am one to do, it is time to run some numbers. Gate houses from what I have read for cities and castles had garrisons ranging from 4-16. With 4 gate houses - one at each end of two bridges - this means that I would think that a full complement in peacetime would be 64. This would allow the minimum of 4 per gate house with 48 left to handle all the other tasks associated with running the fort. This sounds reasonable to me.   The basic formation for the Byzantines was the 200 man banda. The Tagma size I use for the foundation of Gondor's army is 1,000. Now, Dover castle was massive by medieval castle standards (35 acres for Dover verses 2-6 for most others) and had a regulation garrison of 174. Which means that Cair Andros could probably do quite well with a full Banda of 200 as a garrison. Also due to its extreme value, I would not be surprised if it served as the regional headquarters for a Tagma. Which means that 200 would be the base with possibly 1,000 available for reinforcements.   If you have read the section on castles, you will have already read that fortifications of medieval societies were functionally works in progress. Which means that the original core structure would probably be set up for a relatively small number. Then it would have expanded to the point it could hold the full 1,000. But at the banda would probably be the regulation strength, the lodgings for the full 1,000 would probably be tucked into less than ideal sections plus tents. Other houses would probably obtained by booting out the civilians in the fort. Which we can assume would be there given the trade that would naturally be flowing through here. If nothing else they would set up shops meant to service the travelers and traders passing through and service the needs of the garrison.   This civilian and military role would likely mean that in real life, Cair Andros would actually be the core of a local town, possibly a big one. Most likely there would be a civilian and military section of the fortified island in the center. Then as trade continues to grow in the area, so too would settlement on the banks. Remember those palisades I mentioned before? It would not surprise me if these became the starting point of an external wall around the urban development on the banks of the river. This would probably slowly grow into a standard medieval castle style curtain wall. Which would then provide a suitable base for all 1,000 Tagma troops to be effectively involved in the defense of the city and allow for the navy docks to be relocated off the island so they do not compromise the defenses on the critical crossing point.