There are 4 types of troops in Civilization War: Infantry, Archers, Cavalry and Siege Engines.
Training each type of these troops requires a relevant training Barrack building within the Castle, and the higher the level of your these buildings, the stronger troops you can train.
For each troop type there is a unique national kind, and such unique troops are marked with the civilization's symbol at the training screen.
Please bear in mind that even after selecting a specific nation, other nations' troops will not disappear from the training menu.
However, to be able to train those, you'd have to first change your nation and level-up your Barrack, when necessary.
IMPORTANT! After changing the nation you will not be able to train the troops of the previous nation anymore.