John Harbaugh -- Unloading Baltimore Castle ... For a Royal Price

With all of the trade rumors floating around this off-season, we expected ONE of the Harbaugh brothers to be moving ... but we didn't expect it would be the one in Baltimore.

We've learned ... Baltimore Ravens head coach John Harbaugh has just listed his sprawling 2-acre castle-esque property in Baltimore ... and he's asking for a cool $2.1 million.

Problem is ... John paid $2.34 mil when he bought the place back in 2008, according to our friends at Realtor.com ... so there's a damn good chance he could take a hit in the pocket -- unless a bidding war breaks out.

The house is huge -- 8,000 square feet, 6 bedrooms, 7 bathrooms with a wishing well and a fire pit in the backyard. Plus, the estate is only 25 minutes from the stadium.