To enter you need to talk to a woman up after the stairs and demonstrate right spell that she need. If you dont have it she can teach you only for 30 gold.
Hint: After she sees you she start talking; you can listen to her and wait or press E to start other dialog. First way you can get strong spells (like Fire Atronakh or Fear) and high mana needed (only for 30 gold), second way you can get Heal Target spell (translate from Russian) with low mana needed (same 30 gold). Thank you.