If an opponent attacks a battlefield I control, do I still have control of it, and can I use abilities like Ornn's Forge when defending?

Ruling: When an opponent attacks a battlefield you control, you retain control of it while it is simultaneously in a contested state. You can use abilities that require control of the battlefield (like Ornn's Forge) when defending.

Nuances:

• A battlefield can be both controlled by a player and in a contested state at the same time