How do I setup my defensive army in PvP Arena? (browser)

Upon accessing this building for the first time, you will be asked to assign your defensive army to exclusively participate in the PvP Arena. An AI will control your defensive units when someone attacks you, but your defensive army can be changed at any time through the Army Management button.

Your PvP Arena's defensive army is displayed with a yellow frame and a golden tower icon, while your City's defensive army is shown with a blue frame and a golden shield icon:
Just keep in mind that if you remove your units, you will be removed from the PvP arena, and will lose all your progress!