Alaric's Apartment is the place where Alaric Saltzman lives. It was first seen in "Bloodlines" as Alaric left and had a flashback remembering Isobel.
In "Know Thy Enemy" when Maddox kidnapped Katherine and Alaric and brought them here. Katherine awakens in it after being kidnapped. She saw Maddox doing some spell on Alaric. She tried to escape, unsuccessfully. Then it was revealed that Klaus is now possessing Alaric's body.
In "The Last Dance" Klaus compelled Katherine to stab herself and not to escape the apartment. Since then Maddox, Klaus and Katherine live here. In the same episode, Maddox protected Alaric's body(to protect Klaus) from being hurt, because he went to fight with Bonnie.
In "Klaus (episode)" Damon and Andie came to rescue Katherine. They gave her bottle of vervain, which she drank, to prevent further compulsion while Klaus was out and Maddox went to find Greta. When Klaus came home, Katherine was dancing. Soon after Greta and Maddox arrived with a box to transport Klaus' spirit from Alaric's body to Klaus' body that was kept in the box. They cast a spell and soon Klaus was free.