Tactics for the Castle.

Finding such neutral creatures is never a problem, even in the first week.

Hmm... that's interesting. How many level 4 creatures are there at 200% difficulty? Somewhere around a group (10-19). This means you need to recruit at least 3 such groups of monsters in the first week. To do this, you also need to gain diplomacy skills, reaching the expert level, and have enough money to hire the monsters... I wouldn't say it's such an "easy" situation.