When you try to upgrade your mobs their skill, you can, if you have enough exp, upgrade them as far as your exp lets you.
If you try and upgrade your tower however, you need to click each upgrade separately.
So, my suggestion is simple: If you want to upgrade your tower, you can upgrade them instantly, just like mobs.
If you try and upgrade your tower however, you need to click each upgrade separately.
So, my suggestion is simple: If you want to upgrade your tower, you can upgrade them instantly, just like mobs.