I hope you are aware that the recommended hotels are in a rather "better not go" area of Frankfurt, especially when you return to the hotel later at night. As the area around the Hauptbahnhof is interesting by day but also a hang out for junkies, drug dealers and and it is Frankfurt's red light district...
Take a look at this one
http://www.booking.com/hotel/de/hote...tml?aid=306227
or this - or search hotels near Konstabler Wache. The closest Hotel to the location would be Best Western Scala
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