I finally started using my strastegic map spells, as I had been in a nasty habit of not thinking about them at opportune moments.
So this time when an umberdroth came waltzing into my territory, I remembered. So I casted freeze, the pillar of flame. Then...another pillar of flame in the same season?
Hmmmm.....he was 5 squares away from my city (which could not have defended against a full umberdroth), so I simply cast two pillar of flames each season and killed it before it got to me.
Is this intended? Seems a little OP, though I will say it cost a crudload of mana, so maybe it is ok, balanced by mana cost, but I wanted others feedback on this.