[Cohen]

The difference is that a spell cost gems, and requires a mage, and you can cast it anywhere (a spellcasting range could be nice ... like you can cast a fires from afar only to a 5 province range, or similar).
War Engines are in that province only but cost gold and resources and are for sure more easily destroyed.

For sieging spells, there's Crumble, that's very effective because kills people inside the castle.

Another thing I'm missing it's more espionage and secretive/saboge acts. Poison water sources (like wiping out the supplies), steal technology or items and such.

In sieges it could be nice to put the "honourable surrender" stuff (you surrender the fort the very first turn of siege as order and your army is moved away without fighting).