When finding the best unit to make in each civilisation, look toward its attributes. That is, its advantages and disadvantages. Looking at Choson's, it's obvious that towers and legions should be created (but not priests, as it is disallowed in Choson War).

The attributes are:

+80 Long Swordsman and Legion Hitpoints

+2 Tower Range

-30% Priest Cost

In Rise of Rome, another attribute which isn't exactly civilisation specific is the technology 'Logistics', where one barracks unit counts towards half the population of any other unit. However, it does depend on whether or not Logistics is available in the government centre. This makes units from the barracks even more powerful in Rise of Rome, as you can have well over 400 units per player on the map at one time.