How do I easily separate different siege/army units?

Hello, I'm new to the game and I'm finding myself having trouble separating my offensive units like knights and such from my trebuchets.

What usually happens is I set up a siege at a base and then end up always accidentally selecting the trebuchets along with my knights when I'm moving into the base causing my trebs to pack up and effectively become useless. I know there is a hotkey when selecting units to select only military or only workers, but how do I go even deeper into the unit separation?

Thanks!