In my opinion, the magic doesn't need to be changed; it is very well designed, unless perhaps a few minor adjustments are made...
Here is what could be fixed:
1) After all, allow the construction of all creature buildings
2) And add upgrades, but in such a way that only one upgraded building can exist at one level. Meaning you can have both basic units, but you can only specialize in one.
3) Adjust the balance between castles... because the Necropolis castle is just too strong, while Nature is still weak(don't hit me, Nature lovers, but it seemed to me that this castle is indeed a bit weak compared to others)
4) Add various buildings such as: " For example, it would be nice to be able to sell accumulated artifacts, various parchments and scrolls that are no longer needed. Why carry them around in such quantities! Better to sell... especially considering the constant need for gold. " author:Pandora.
And others similar to those...
I am ready to hear constructive criticism...