Hey I have some questions about this card :

1-If i attack with a monster than i use this card to special summon a monster. Can i Stop or change the cible of the attack of the first monster ?

2- is my oppenent Obliged to summon a spellcaster if he has one in his Deck ?

3- If the oppenent do not summon a spellcaster should i see his deck ?

1. No because Magician's Circle has to be activated when you declare an Attack Target with a spellcaster monster you control.

2. Yes your opponent has to summon a spellcaster type monster from his deck with 2000 or less ATK points.

3. Yes because your opponent has to summon a Spellcaster monster, it's not an optional effect.

Regarding the first question, it depends on whether or not your opponent summons a monster. If he does, then you can either continue the attack, not attack with the monster at all, or change the attack target.