Squattering people is better strategy than running. Also, some things, like doors, can also be destroyed.
Conjurers can summon up to four scorpions with full mana. Scorpions do not count as population, so you can have 'em as much as they can fit the map. Same rule applies to orcish spiders.
Elemental is the strongest unit humans can create, but still weaker than orcish demon. He compensate that slight weakness with increased range of attack.
Mages can create both scorpions and elemental, which do not count as population, but takes time for mana to recharge.
Entering the cave means you won't get any extra reinforcements (unless raising dead or being able to summon).
Catapults are still the beast weapons when they hit the target. But protecting them is always a must.
Always the best strategy is to have archers right after the knights. Knights ahead can always intercept enemy catapults since they can not return fire at short range.
Deamons summoned from hell by orcish mages are the strongest creatures able to wipe entire cities out when in group. As all summoned creatures, they disappear when magic hits the bottom.
Extinguishing every trace of human existence will open your gate to invade & conquer other worlds in the future.