Nora is neither the protagonist nor the antagonist in A Doll'sHouse.
Nils Krogstag is the protagonist. He is able to blackmail Nora because of a loan document in his possession which bears the signature of her husband which she forged. Krogstad threatens to reveal the crime unless she persuades her husband to reinstate him in his position at the bank. Her husband Torvald is thus the antagonist, because he is adamant against rehiring Krogstad. Nora is torn between these two strong, determined men.
The dramatic question is whether Nora can persuade Torvald to hire Krogstad and what will happen if she can't succeed. The climax comes when Torvald receives the letter from Krogstad telling him about the forgery and what he will do with the document if Torvald does not relent. Although Nora is not the protagonist or antagonist, it is her story, everything is seen through her point of view, and the most important event in the play is the character change she undergoes when she sees her husband as he is and their relationship as it exists.