I was there (Frankfurt Marriott Hotel) a couple days ago. Its 32 EUR/day. Plenty of spaces, but some are numbered, some are not, some are obviously disabled spaces and some has these blue squares next to them. I parked in a different numbered space each day and wasn't towed or ticketed. The garage isn't part of Marriott and you have 10 minutes to leave after paying. I could have paid and put in on the hotel bill, but you are wasting 1-2 minutes of that 10 minute allowance. I paid with credit card at a machine just outside the hotel lobby.