I had a two-night stay in Mainz at this fine little place. You would never just happen past it in a random stroll around Mainz. It's located in an upper-end, quiet residential neighborhood that is just 10 minutes or less on foot from the Mainz Hbf train station, a walk that's flat and easy near the station and takes you uphill for a couple of blocks as you approach the hotel. The REAR station exit allows for the most direct walk.
My single room (€62/night w/o breakfast) was surprisingly roomy for a single in Germany (where singles are often large closets.) Private bath, a wardrobe, a small table + chair, a night stand, TV, a small dresser, and a balcony with a small table + chair with a treed view were included. The balcony-side wall was full of windows that brought the outdoors in. Everything was spotless and functional. No coffee-maker :( but I knew that going in and had chosen to just get breakfast at the station (where Yorma's, McD's, etc. open quite early.) Breakfast is available for €15; the breakfast area looked comfortable and tidy, and online reviews are good. I slept like a baby at this place. The reception has limited hours. Staff was really friendly and helpful. Adjacent to the hotel is a large park-like are with walking trails and also a sleep-study clinic, which I think attests to the quiet environment. I never expected to find a place like this so close to the main train station in Mainz, which by its nature is a non-stop busy, noisy environment. My sole complaint is the spiral staircase that led to my room and the awkward angled-step that confronts anyone wanting to go up from the staircase entrance. I must have tripped over that thing 3 times. Use the elevator. I booked using booking.com.