The card text makes the special summon mandatory for both players, so if one player doesnt have a monster zone available the card cant be activated. Why youre able to summon a monster if the opponent has no monster to summon is beyond me, but whatever. Text should be "each player CAN" which would make it optional
I noticed that magician's circle doesn't work when I attack with a just summoned spellcaster monster, even if magician's circle was already set turns before. i.e: 1st turn I set magician's circle and summon gravekeeper's recruiter; 2nd turn I tribute summon gravekeeper's oracle and attack with him, trap card doesn't activate; 3rd turn I attack with oracle and trap card works. Is it meant to work like that?