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weakening unimproved neutral armies through repeated defeats

A tactic known as hit'n'run: attack, cast a spell (a damaging spell or Berserk), and retreat. It is usually used on single-player maps against enemies that cannot be defeated in a normal battle, but need to be weakened as much as possible. This is repeated until the enemy army is reduced to a size that can be handled in a regular battle.

how to create a hero without an army?

It is possible, but not on every map – only on those where there are Seers or Guardians that require you to bring N specific creatures – the game allows you to give up ALL of your creatures in such a situation. As a result, the hero without an army receives the maximum number of move points the next morning. This is possible on quest maps, and in "Wayfarer" it is even necessary according to the author's intention, so that you can reach the required location on time.

the same actions, first attacking a town, then a hero on the road – the result is different!

The number of turns for siege engines is also taken into account. When besieging a town, a catapult is added to them. Another factor is the terrain on which the battle takes place.