Tactics for the Swamp (Fortress).

Well, building a Castle isn't always possible, I agree. :) I presented an ideal scenario – what you try to achieve by any means necessary.
Still, based on my gaming experience, the main problem for Cavaliers + Castle in the first week is wood. We need 5+10+20+10=55 wood and 1000+2000+2000+5000+2500+5000=17500 gold to implement the plan. If we're lucky and there are 3 wood piles at the lumber mill, we get an average of 3*8=24 wood from them + 2*6=12 wood from the lumber mill by the end of the week. In total, we are guaranteed to get 36 wood. In principle, with a more or less favorable situation, finding the remaining 9 wood and scraping together 17500 gold is theoretically possible. The main issue here is a continuous flow of money, increasing towards the end of the week. Any money-generating hero can help with this, again, if we're lucky.
In general, it's a matter of luck. =) Before the game, be sure to visit the Swan Pond at your home. :)\
P.S.: The problem is that when you manage to implement the plan, there isn't always enough money on day 121 to buy an additional hero and 6 Cavaliers.