The most useful monster.

A Vampire is easily defeated by a Crusader.
And he isn't that strong anyway. Archers can also deal with him quite well.

Perhaps that's true. A lot has changed in the latest version!
P.S. A friendly piece of advice: don't limit yourself to just one aspect of the game. Otherwise, you might regress in your ability to play. A good commander always knows more about the enemy than the enemy knows about them! This has always been your advantage.