YA Fantasy teens join army which has three branches

[Anthony Corrado]

I'm pretty sure about the three branches. I recall the main characters living in a castle for the training. The castle has a courtyard int he center with some sort of tree or garden. At some point they go to explore it and find a hidden staircase the goes into underground tunnels. I read this in june/july of 2017

[Kris]

Anthony, what's unique about each of the army's three branches? Who is the enemy?

Can you describe this fantasy world?

Can you tell us more about the main characters - their genders, ages, rich/poor, powers/skills/interests, memorable traits? How does each character come to be at the castle?

[Astra]

It could be Shadow and Bone by Leigh Bardugo.

https://www.goodreads.com/book/show/1...

The Grisha Army has three sections, Corporealki, Materealki, and Etherealki. They train in the Little Palace in a city called Os Alta. I'm not sure about the courtyard.