We are a group of new players. I feel that I miss some important properties of the levitate spell, as it proved extremely powerful in the last encounter in Cragmaw Castle in The Lost Mine of Phandelver.
The party caused an alarm to ring, which caused the entire final group, with bodyguards, to prepare an ambush when they stormed the last room.
The Doppelgänger
managed to immediately down a player in the surprise round. After the rest of the enemies chucked away quite a bit of health as well, it almost looked like it would become a full party wipe.
The wizard cast levitate on
The Doppelgänger
(and it failed the saving throw), which effectively disabled it. Thanks to the 10-minute duration and the wizard carefully saying that it "should float riiiiiight in the middle of the room" (so it couldn't grab any wall object to pull itself down) it wasn't able to do anything during the entire rest of the fight.
From my understanding, it was not allowed to re-try the saving throw on its next turn. The other enemies were not able to break the wizard's concentration either as she was hiding behind the rest of the party in a small doorway which blocked the enemies from reaching her.
I'm happy the party managed to survive the encounter, but feel that casting levitate on any melee-based enemy could quickly become a effective, but boring, strategy. Is there anything in the rules that I've missed?