Endurance (or should it be called Addiction?) (ones that require a big time investment more than anything else - e.g. build 1000 temples)
Difficult (even on low difficulties these might be difficult e.g. Win a domination victory without losing a single unit)
Insane (near impossible - e.g. beat a tank with a spearman)
Beat tank with spearman: Find out the chances of a fully promoted healthy (or Japanese) spearman against 1HP tank (e.g. 1000:1). Arrange such a fight 1000 times and hope. It's either actually impossible or it belongs in the "endurance" category if you ask me
