So here we have them... Create Mountain, Lower Land, Raise Land... all extremely powerfull spells that allow one to seriously, and by seriously I mean SERIOUSLY dictate the flow of everything around your lands. Currently I have a load of mana lying around you would not believe... what stops me from creating a double mountain ringed "island" that is only accessible through two holes, opposite of each other, with two good stacks blocking the entrance, and 1 shard type of each inside and then cast my spell of Mastery? There's no way you'll be able to get there, unless you, the AI, uses Lower Land to create pathways.
AI is located on peninsula, his main city at the back. No way for me to reach it other than through his armies... but his vision range is limited to 6, max... so I send two armies towards the sea near his peninsula, and create a landbridge and stomp him. From behind. The final landbridge is created in one turn of casting ( takes 300 mana), and my troops are already halfway over before the turn has passed.
Oh noes, the AI is coming towards me over that land-bridge that connects the contine... *SHAZAM* no it doesnt.
Look, a massive army approaches my capital... * casts create mountain a few times* detour!
Terraforming... take into account that players can and will create their own fortresses, islands and Mt. Doom.
I personally vote to put a cooldown on those spells.