This is my suggestion:
Add a new building:
The Summoner’s Tower
Different levels unlock different demons…
You “research” or summon demons for a high gold/time cost, and these demons can be placed along your path as guardians. The demons inflict damage but cannot obstruct the path because they are ethereal (without physical bodies).
So most units wind up passing through them. But a few units like Kings, Frosters, and Pyromancers can kill them.
One fun thing is they could be included in the chamber of fortune. So you could open a chest and get a new demon to place in your defenses.