I didn't know that - at my level it was 400 swordsmen (if the Wiki is right, I don't remember exactly but I did it and I know it wasn't Heavy Infantry).
I agree that is ridiculous for all players in that level bracket to be expected to do that. It so happens that I have the PvP buildings but they are just hanging around I don't do much with them and I consider them disposable. PvP is expected to be completely optional.
Yes of course the event chain itself is optional if you want to look at it that way (and that's often my point of view) but if you have started it by the time you have reached that point you are heavily invested and compelled to continue it.
A very bad choice by BB.