Containing the Innenstadt of Frankfurt is the Green Belt (Grune gurtel).
It is spacious park shaped like a C that holds the city up against the river, but it’s modern function is not without an interesting history:
Ringing the original fortified city of Frankfurt (up to a few hundred years ago when European towns and cities needed walls to keep unfriendly neighbours out (like the French)) were moats, high defensive walls and towers. Once it was clear the French would not siege and bombard Frankfurt again, and peace calmed 19th Century Europe, the fortifications were converted into what is now known as the Green Belt.
The Green Belt is a succession of parks with footpaths, leading to ponds, gardens, statues, fruit trees and forests. It has been recognised by the United Nations as an example of sustainable city development and is quiet place to rest the feet and eyes.
It is easily accessible from the city, and once in the Green Belt, surprisingly easy to navigate around the city. It takes one hour to walk the Green Belt from end to end. On a bicycle you can do it in perhaps 10 minutes along the pedestrian paths - that’s at break-neck tourist-terrifying speed.
To walk the Green Belt, start from Willy-Brandt-Platz, at the foot of the Euro symbol. At any time you can head back into the city, or to one Frankfurt’s suburbs, e.g. Westend, Nordend, Bornheim, depending on where you are.
In the Green Belt are numerous old and modern statues and sculptures.
There is a children’s playground on the eastern side.