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I actually tried a fast development strategy for the first time yesterday in Might and Magic, playing as the Necropolis faction. I followed Fizmiga's tactics, but with some adjustments. Basically, I quickly built Vampires and Liches. I started with Galthor. On the third week, my heroes were defeated, and it was all because I spread my army too thin. And by that point, I didn't have enough money to buy a new army. Therefore, if the map is larger and the enemies aren't close, you should first strengthen the financial aspect of your Castle (the Capitol, and with it, the Castle itself), not hesitating to sacrifice the first week completely. Although, I think that with the starting army plus a purchased hero from the tavern and their army, you can easily defeat weak neutrals and capture a couple of extra mines. And only then should you start building creatures, emphasizing that you shouldn't build all of them at once, but rather consider the amount of resources you've gathered (on a high difficulty level) and the creatures that are most important to you.
In general, each "Heroes" player develops their own tactics over time.
Added after 2 hours 59 minutes
P.S. I did manage to complete that Might and Magic game as the Necropolis faction