Storm the same escalating fortification across five levels -- pick your army each time, break the gate, take the keep.
You command the attacking army. Storm the same fortress across 5 escalating levels -- break through the outer gate, then the keep's own gate, and defeat the defenders (and the King himself, waiting in his tower on level 5) to win. Survivors carry over into the next level at full health; buy new units and siege equipment with your budget before each battle.
Score adds up across all 5 levels: a base amount for clearing each one, a bonus for beating that level's target time, and a bonus per surviving unit -- then, once you beat the whole game, your total is multiplied by your difficulty (Normal x1, Hard x1.5, Impossible x3) for your final submitted score. Survivors are units still standing at the end of level 5 (siege equipment doesn't count). Heroes are survivors who've fought since level 1 -- a unit bought or added in a later level doesn't count as a hero even if it lives to the end.