New Player! How do I stop my mangonels from hitting my own guys?

I like to play as the Persians so I usually go with an army of knights and some mangonels (to counter those pesky pikemen). My army charges in roughly this formation:

🐎🐎🐎🐎🐎🌭 🐎🐎🐎🐎🐎🌭 🐎🐎🐎🐎🐎🌭 🐎🐎🐎🐎🐎🌭 🌭 = mangonel 

However, when my mangs fire at the first line of enemy soldiers they often end up hitting my own knights!

Does anyone know a good strategy for controlling this sort of army composition? Thanks!