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Without tactics, you won't get anywhere.
A strong warrior encounters a strong mage (or even another strong warrior without tactics). You have the first turn, you position your units, cast a mass haste spell, and thin out the enemy.
The mage also needs the "Tactics" skill to neutralize the enemy's "Tactics" skill.
Or, here's an example from the beginning of a game. You have a Tower, gremlins, gargoyles, and a small melee unit. You see a dwelling of giants and attack it. The giants move, gain morale, and start attacking the gremlins. The game is ruined.
But with tactics, you would have protected the gremlins with gargoyles and defeated the giants.
The same applies to a Castle – when many archers take cover behind angels.