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Usually, I do the following: I build the main building in the castle simultaneously with the recruitment buildings. To do this, I sacrifice fortifications, as they are not very useful at the beginning, as well as magic. Later, I complete them.
In battle, I act as follows: (in general, the strategy is based on vampires)
1.1. With the initial army, I attack only with the strongest unit, but not with ghosts.
1.2. If the strongest unit is ghosts, then I defend.
1.3. Among the creatures, I try to get vampires as quickly as possible, and if I succeed, the vampires rush into battle at the forefront (with a small army). Zombies and others rest and accumulate numbers for the middle stage. Except for skeletons, I can't seem to keep them, but that's what Necromancy is for!
2.1. With a medium-sized army, I try to protect the vampires, because they don't have time to recover in a situation where both they and the enemy are in roughly equal numbers. Instead, I have to sit in defense, as in the situation with ghosts.
2.2. In this case, ghosts (or zombies, but only advanced ones) go as cannon fodder, drawing attention to themselves; fortunately, ghosts are saved by their slight incorporeality, and zombies by their toughness.
2.3. The hero and archers shower the enemy army with whatever they have, after which it perishes.
3.1. By the time a large army is gathered, the vampires (usually "Greater Vampires") accumulate in numbers, and if there are more than a hundred of them, they can be safely sent into the thick of the enemy. With such a mass, they successfully regenerate themselves.
3.2. They need to be provided with support, and not only moral support. Ghosts are suitable for this role if they have accumulated from the second stage. The others won't be able to keep up with the agile vampires. This is especially true when it comes to besieging an enemy castle (you'll understand why).
3.3. If the enemy has a high-level creature, it poses a threat to the vampires. I try to eliminate it with the Necropolis's not-so-useful dragons, along with archers and the hero's magic.
3.4. Vampires are most likely already surrounded by several squads in the enemy's rear, and here a "cloud" of archliches is useful.
3.5. Zombies guard the archers, and Death is in reserve.