Caroline's place was probably as central as you can get. Just outside the apartment is Stefansplatz, and the cathedral is just around the corner (you'll get there on accident). The metro is just outside the door, so the guests are immediately able to access the rest of the city with ease. There was everything you would need from the apartment (functional kitchen, bathroom, and living/bedroom). It's incredible! I would highly recommend this to anyone looking to enjoy Vienna.