My stay at the Astor & Aparthotel was awesome. I walked from the train station, which was a little under a mile. The staff was incredibly friendly from the start. My room was small, but fine for one person. I had a balcony with a view of the cathedral. The area seemed safe, but they have a strongly recommended policy of leaving the key at the desk (the hotel name and room number are on the key chain, so they don't want you losing it). It seemed a little weird, but I never had any trouble getting my key back. I would absolutely stay there again if I'm ever in Cologne.