I was playing a game and Gandalf got charged and knocked down. He survived. My opponent won priority gandalf gets up can he cast this turn or not? We played that he could but i was wondering if theirs a definite answer?

if he is free to move (not in comabt) he can get up and use magic provided he has enough movement left to get up don't forget if he is on the floor he has almost no line of sight and if he gets charged he cannot cast anything

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If Gandalf does have the priority then he can cast, he won't be very mobile, but he can cast to his hearts content. If he's re-engaged before he can cast, that's another scenario, then his spellcasting is to say it mildly "limited".

If you have the points, always put your spell caster on a mount. The extra mobility nearly eliminates their need to worry about being knocked over.