Two other Ideas I heard (and also adapted on proposals on different threads):
A) Pitfalls: A defensive tool that can be built by Villagers. Pitfalls can kill one enemy unit stepping over it immediately. They are invisible to the opponent until used or destroyed. And your own units can walk over them. Pitfalls could be countered by Mangonels, Villagers (converting them to your own like Farms), Rams, BBC and (when hitting them by chance) Trebuchets.
(It’s debatable if a single Pitfall should be able to kill an entire Elephant or if it needs more)
Pitfalls would then be especially effective against Knights due to the high pop efficiency of the Knights.
But it would still be an interesting interaction as if you have sonciderably higher amount of Knights some of them survive and then can be able to raid the most likely oterwise unprotected Villagers.
B) A better or entirely new Power Infantry Unit in the Castle Age. A unit that soft-counters Knights (but also has some resilience vs Archer fire). I for myself would like to have an entirely new unit as the current militia line design is just too far off from being able to do this. I also think to make this approach acceptable for the community, the new infantry unit must have some interesting properties, especially something that can be used to give you an edge (eg micro potential).
In my design I intentionally increased the cost to get these “Guards” rolling so the Knights have some time to deal damage before these things are out. Preserving the initial early castle age Knight powerspike.