Tim17 is right – at maximum difficulty, the castle only needs wood and ore. You can immediately build level 4 creatures and a citadel, and if you're a little lucky with money and resources, you can even build a castle. At the beginning of next week, upgrade the archers, buy everyone except the swordsmen, and go forth to conquer! There might be trouble with the monastery, but with an abundance of wood, level 6 cavalry can be built quite quickly... and if there's also an external stable near the castle, then champions aren't even needed – the upgrade is free!!!
With a decent respawn rate, two weeks is enough to create a decent army.