Can only be built in the capital. At level 10, you can upgrade your Praetorians to Centurions. This is a one time upgrade and lasts for the game round.
Centurions gain strength in numbers. For every 100 Centurions trained in a village, their defence is increased by 0.05%. The bonus is maximised at +5% strength if you have 10,000+ Centurions trained in a village.
Additionally, you can use the Officer's Quarters to host Military Parades. These Parades boost morale, and for 24 hours, Centurions move at 9 fields/hour. However, they will become exhausted, and move at only 3 fields/hour for the following 24 hours. You can throw Military Parades in any number of your villages; you use the Officer's Quarters to select which ones.
Lv1 to Lv9 - No effect
Lv10 - Option available to upgrade Praetorians to Centurions
Lv10 - Option available to host Military Parades
Lv1 rough cost
500 wood, 650 clay, 850 iron, 250 crop
40 minutes to build
Lv10 rough cost
48000 wood, 60000 clay, 82000 iron, 22000 crop