Was trying to use them to get to a minor city that was completely blocked-off from normal access by ocean & mountains.  Lowed several mountain squares and raised ocean, trying to blast a path to the city. 

Mostly, the game would not let me step onto my new plains squares even though it looked like they ought to be passable.  Bug ?  (I think I could get in to some, but not all, after exiting & restarting, but I might be mistaken).

2nd, it would not permit me to cast onto squares with the city's area of influence.  Guess this might be a deliberate design choice, but I thought it ought to be permitted.