A place to recruit special units. Through upgrading can increase the training speed. Resulting in faster training.
Here you can recruit your special tier 5 units. For each different civilization the units are different.
For rome, you can recruit Armored Knights and Gladiators
Persia, Persian war elephants and muslim calvary
China, Chu-no-ku and Mangudai.
Egypt, Anubis Guard and Chariots.
Each level of Castle increases traning speed by 1%.
To build one, you will need Town Center level 17 and Barracks level 17