help me solve a priority issue

[Rhengar]

So im in the process of making a pve game where i try to mimic a flash game called epic war, Ive gotten very far already but stumbled upon an annoying issue i cant seem to fix. That is, my units are working with 0 collision size and flying so they can stack onto eachother. Now my units spawn behind the castle and move towards the enemy castle(castles have big collision size cz theyre big) But since units are flying with 0 collision they can move trough the castle, which is wat i want, at least for my own units. The idea behind that is that if the enemy overpowers you with a big wave of units, u can stack ranged units behind the castle to destroy them, now heres the issue. Whenever an enemy sees a unit which is behind or moving through the castle, they immediatly start to walk towards them, so the point of the castle blocking the enemy is useless.

https://cdn.discordapp.com/attachments/918438231614316545/940914571273723934/unknown.png

https://cdn.discordapp.com/attachments/918438231614316545/940914830674636840/unknown.png

i hope the explanation makes sense, if anyone could help me it would be super kind!