Heroes of Might & Magic III: Horn of the Abyss

Why can't you create a monster that occupies three cells? They made Dread Lich, which occupies 6 cells, and then you become invincible. You could create a super catapult that occupies three cells or something like that...

1. What is the purpose of it? (I mean, a monster that occupies three cells).
2. This would deviate from the original Heroes of Might and Magic III.
3. It wouldn't be able to stand at the top or bottom of the castle.

6 Dread Liches and you're invincible? I've never heard of that. Moreover, Azure Dragons are stronger than Dread Liches.