I've been asked many, many times how I manage to rotate roof and base parts since all we can do is swivel them around. There are two easy ways to do that:
Place a roof or base part on any connector attached to another basic part. Then rotate the connector until you've reached the desired angle. Now you have two choices, you can either drag the base/roof far enough away with the shift or alt key and delete the connector or simply copy the base/roof and a copy will be left in place. You can then move the connector again and place another tilted roof/base to create more advanced creations. This technique allows for creating circles, arches and spirals using nothing but base blocks.
Note: Doors are considered detail parts and will automatically reset their position when you accidentally hover the mouse over an arrow of the base part.
Using the Rotation Mod
DVDMaster was kind enough to make a Building Editor Mod adding an additional rotation ring to the base and roof parts when you hit the tab key. You can install it like other mods by placing the .package file into the Data folder of your Spore installation directory.
Note: Currently, the mod only works on the PC.