Let's make a simple calculation. The map has 100 territories. Castle has 37 territories and every territory on castle has average of 300k armies. At least. That is 11M armies to clear. You have 100 territories to get income, hence each territory needs to generate 110k gold. Turn 1 territory makes 1 gold, turn 2 two gold, turn 3 three gold etc. until you decide to not spend gold for cities and save it up for armies. Let n be the turn when you stop building cities, you need n+110k/n turns to get enough gold. Using the sacred Japanese technique of derivation, the best turn to so stop is turn n=331. This means you need to spend at least 331+110k/331=663 turns to clear the level.
And yes, I assume that all armies on castle are in a single territory and that you start the level with all but one territory. Except the amount of turns to be more than 663.
That being said, I fully support this idea.
Edited 5/18/2021 21:30:12