Castle (STW building)
It doesnt produce units.
A Castle is required to allow you to start building other buildings which produce troops. It is a last resort defence when your region is invaded. Retreating to the castle in the face of an attack reduces the attackers' options to a lengthy siege or costly assault, but removes any possibility of your forces retreating from the province.
There are four levels of castles in Shogun Total War, but they all perform the same function. They are the bases for armies and the visible signs of the daimyos’ power, honour and control of provinces. Without a castle to act as an administrative centre, no other military structure can be built in a province.
The simplest (and cheapest) castle type in the game is the castle (castle 1). All other types of castle are developments of the basic castle. A castle is roughly the equivalent of a wealthy landowner’s fortified manor house. At the other end of the scale, the citadel (castle 4) is a truly awe-inspiring structure equal in scale and grandeur to Osaka Castle. In all probability, there won’t be more than one or two citadels built during the course of a single game of Shogun: Total War - Gold Edition.
As well as their more obvious defences, Japanese castles were also designed with tricks and traps to defeat ninja assassins. All castles add to the honour and prestige of their owners. They are visible symbols of wealth, power and permanence and as such send a powerful message to friends and enemies alike just by “being”.
Each type of castle can have a number of military buildings and functions attached to it, as described below. As a general rule, the larger and more prestigious a castle is, the better the quality of its associated buildings, and the better their products. A small stockade, for example, can only have the most basic type of each building attached to it, while the larger castles attract master and legendary craftsmen and sensei to work in them. These highly trained individuals help to train better quality troops and a greater variety of them too.
In Shogun: Total War - Gold Edition, you’ll probably find that it is wise to create one or two large castles within your domain that act as specialised “centres of excellence” for one or two kinds of fighting unit, rather than create a castle in every province and hope to make them all perfect. Remember that it’s quite easy to run out of money: harvests and taxes come once a year, but the money can be spent all the year round! Remember too, that castles and the military buildings can only support your efforts to become shogun. In order to win, you’ll need soldiers, not just the places to train them!