Hotel Karolinger in Dusseldorf

I am writing to express my extreme dissatisfaction with my stay in your hotel room. The experience was unacceptable, especially for the price of €84 single room small window with no view .
The room was freezing cold and visibly dirty dusty (a major concern as I am allergic). The heater is loud and non-functional. I was unable to sleep all night.
Furthermore:
The room is too small less than 10msq and does not match the website images.
The flexible shower hose is broken and leaking.
There was no receptionist/staff available on site, which is unacceptable for a hotel.
When reporting the issues, I was absurdly told to change the bedsheets myself

I booked this hotel in advance. I arrived at 4pm and I took out the key from the mail box. There was no reception. I climbed the stairs and when I opened the door of room nr16, I was shocked by the smell of humidity. The wall had yellow stains. Being the room small, made it even worse. It was impossible to breathe. I called the hotel phone number, a lady replied, but she couldn't give me an answer if there was another room available. She was needing to contact the boss. After the 3rd call, and also calling the web site agency, the lady told me that she received confirmation from the owner that there was no room available. To the complain that I submit through the web site the owner replied that they were not able to refund my money because I had use the room and they will need to clean it up. I only stayed 45m at the door of the room, trying to get an answer.