Cities XL includes a number of decoration areas, which work on the freeform filler principle. This means that the area will automatically fill the entirety of an empty space, no matter what its form. All decoration areas are found in the Decorations>Areas Construction menu. To build an area, you have two options: 1. Find any space, enclosed completely (or almost completely) by buildings and/or roads. 2. Define the space yourself. The easiest way to do that is to use roads to enclose a space in the form you want. Later, you could even buldoze the roads and leave only the new decoration area!