when considering the risk of losing a Castle, the AI becomes completely passive. Its main hero doesn't want to move further than the garrison gates (and rightly so).

I freely moved further than the gates.

What kind of "demand" for mines are we talking about if, at most, only two sets are needed? In reality, one set of mines is enough because your Castle is largely built.

The key word here is "needed" - they are needed throughout the game, not just in the first three weeks. I wasn't saying that 100% of them are needed. It's easier to kill the enemy than to capture all the mines, meaning they remain in demand (even if it's just one or two sets). Usually, after a month of playing, you only need gold.

The map is controlled from the Necropolis; you need one secondary hero there, who can intercept everyone who leaves the main hero. In total, you have two mandatory heroes, and the rest are up to you. This setup exists for almost all normal maps.

But that's a matter of personal preference. I had a main hero and two secondary heroes - one defended the Necropolis, and the other defended the capital. Each of the secondary heroes made small sorties to neighboring locations when enemies appeared there, which could take time away from the main hero if he had to go after them himself.