I kinda mentioned that in a post just recently, actually: http://www.textfugu.com/bb/topic/help-please-beginner-is-stuck-already/
Essentially, the location (marked by に) is also functioning as the topic (which is marked by は). Which is to say, the topic of the sentence is “at my apartment”.
The topic of a sentence is almost always a word that’s already playing another role in the sentence – when it’s the subject (which is marked by が) or the direct object (which is marked by を), the topic particle は replaces the particles completely and just stands on its own. In other cases, however – such as with に or で, for example – it joins forces and they stand together as a double particle.