Note
When you build 1 of something, you only build a part of a building; each part takes 1 land. You need 500 castle part for a complete castle. 1 castle part is only 1/250,000 as effective as a complete castle. 250 castle parts have 1/4th the efficiency. When building castles you should consider that a part of those castles will be destroyed when you are attacked. So it does not make sense to build castles unless you can build at least 1500, better 2500 parts which usually requires around 10k land.
Castles are the best Buildings but are extremely hard to build. They are huge and require 500 parts. They are extremely time-consuming and expensive to build but are a tribute to the kingdom's leadership and will have dramatic effects on the population, Laborers and Scholars in the kingdom. A full Castle will greatly increase your study and build rate and will act as five full Training Fields. They provide storage for 100,000 Weapons and pieces of Armor, to keep them from rusting away, they protect 10,000,000 gold and 25,000 Weapons and pieces of Armor from looting by enemy Thieves and they even house 100,000 troops. They also provide a nearly impossible breach defence against attacks for 50,000 troops (increases their attack and absorbtion by 100% - weapons, armor and war machines too) and they increase the Mana your population generate.
A note of caution though is that castles are very expensive, require good construction study and lots of laborers to build. Each castle adds 20k score which makes it more difficult to attack without mutiny for most races. Not only that but when damaged they rapidly lose their benefits so never build just one castle. Usually its best if you can have built at least 5 full castles (that is 2500 land). Many very good players never build castles due to all of the negatives associated with them. One notable exception though are Sprites and Leprechauns who can really shine with a Create Food economy. The castles house troops and increase mana which allows them to eliminate farms completely. This is an advanced tactic though that is not recommended for most players.
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