When you start the map, its like any other map where it only gives you a starting tile, as you build your city, more tiles become available to purchase. Cities Skylines limits to 9 tiles to be purchased in total as your city grows, but there are several mods on the workshop that unlock all tiles (81 tile mod) or unlock 25 tiles (after 9 tiles purchased) which would allow you to play the whole circle.

If you check out the Great Plan collection, you'll find an unlocked version of this map which you'll need to play with unlimited money.
https://steamcommunity.com/sharedfiles/filedetails/?id=2345832488

81 tile mod:
https://steamcommunity.com/sharedfiles/filedetails/?id=576327847

It's very pretty and I really wanted to play! I started it and despite all tiles being unlocked, only 1 tile was zoneable - thus the rest of the map was dead. Not sure what's up, that makes it unplayable.