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It depends on the rules you're playing by.
If there are no restrictions:
1. Necropolis.
Heroes: Galtran, Izra, Vidomina.
Build order: first week - vampires in the Citadel (preferably in the castle), second week - Necromancy upgrade, Death Knights in the castle, if possible - dragons.
Heroes at this time gather skeletons and level up.
Third week - try to take the Utopian.
If you get the Air magic book or wings, take them immediately, grab all the gold, and fly to your opponent. That's it.
2. Tower.
Main hero - Kirr.
Immediately buy several heroes, upgrade Goblins, and destroy everything. This tactic is called the "Goblin rush." If there's a building with the "War Machine" skill, Torosar can be very effective. In general, there are many native heroes with archers, plus there's a skilled archer who can train the others.
By the second week, Giants should be available! Or, if you have a few Naga lairs, you can even try to do without Giants (but it's better with them).
By the fourth week, you need to somehow take the Utopian. And make sure at least someone survives. After that, it will be much easier. Upgrade Titans, and then continue the fight using your main trump card - the powerful Magic Guild.
3. Dungeon. Starting hero - Shakti. Later, he will most likely be the main hero. You can also level up Gunnar and Jezhzhit with Alamar in parallel. An early upgrade of Trolls is desirable - and then go for it. Destroy all buildings, take money and resources. If you put Minotaurs in the castle early on, that's a good start. Next, a very important point. If you see a lair, think about it. Maybe a better one will appear. For example, if your opponent has a Dragon lair, they've almost lost. Take it in the first week. Buy a dragon. Build a portal. Buy a dragon there. By the beginning of the second week, with 4 dragons, you can cause some havoc. If the game drags on, it's not fatal for you - even in the third week, if you build dragons, you'll have a growth of 5 per week (1 lair, 1 from the portal, 3 in the castle thanks to the lair), and that's it. You understand that if there's also a Troll lair, then not always dragons will come out of the portal...
Okay, let's continue. If there are no lairs, you have 1 strong stack of upgraded Trolls in Shakti's hands, and by the third week, a good Minotaur squad will also accumulate.
Next. You'll put Dragons on the second or third week. Don't worry, you have other trump cards. On the third week (if you're lucky), and definitely on the fourth, you need to destroy the Utopian.
Remember that red dragons are reborn, but black ones are not (taking into account heroes with specialization or the ability to learn the resurrection spell).
Don't be afraid of anything after that. Your Minotaurs deal damage comparable to that of level 6 units, and the Trolls (if they survived after the Utopian) in Shakti's hands are a formidable weapon. Two excellent ranged units without a penalty in melee combat are also a good thing.
Remember the following:
1. A normal player's army usually contains Angels from the Fortress and Wyverns from the Hive. Moreover, these are often some of the main attack stacks.
2. All kinds of buildings on the map are almost free experience and money.
3. The first week is for building an army. You'll get the money back.
4. Don't be afraid of anyone. Forget how you used to play. Use tactics.