David and Charles Koch are the principal owners of Koch Industries, the second-largest privately held corporation in the U.S., and are known throughout the industry as the infamous Koch Brothers. Each is worth about $44B and they are known for their enormous political and philanthropic contributions. Though David and Charles are often in the limelight, let's not forget about the two other Koch brothers: Bill and Frederick. Not as wealthy as their siblings, Frederick is an art collector and has a net worth of $4B, and Bill is a businessman with a net worth hovering around $2B.

While the Koch brothers are all notoriously private people, it's not easy for four billionaires to hide all their real estate holdings from the public. Bisnow hones in on five of the brother's most impressive real estate assets next.

David's NYC Apartment

Just narrowly beating out Michael Bloomberg as NYC's richest resident, David Koch's primary residence is his 18-room duplex on the fourth and fifth floors of 740 Park Ave. He reportedly paid $17M in 2004 for the pad in the legendary luxury co-op, although all David himself said was, "I paid an attractive price."

Bill's Aspen Ranch

Just 10 miles outside downtown Aspen sits Bill Koch's 82-acre western estate, complete with a main house, seven cabins and four single-family lodges. In total the property boasts 28 bedrooms, 27 bathrooms, Colorado's largest hot tub, a movie theater, a full indoor basketball court and two fishing ponds. Bill bought the property in 2007 for $50M and has since injected at least $50M more into renovations. Today he's looking to sell the estate for $80M because he's building a 50-building replica of a Wild West town (also in Colorado).

Charles's Palm Desert Mansion

While Charles Koch still works most days in his Wichita office, his prized getaway home is in Palm Desert, Calif. The 7,500 SF mansion has five bedrooms and eight bathrooms and is worth upward of $10M. When home, Charles is often holding political fundraising events at the Vintage Club down the road, where admission can hit $100k per head.

David's Palm Beach Home

Despite living in a dream Manhattan apartment, David also has several getaway mansions to choose from. The most impressive is his Florida beachfront property in Palm Beach, which is thought to be worth some $27M after David spent $12M upgrading it. The mansion covers 30k SF, but unfortunately it doesn't have a privacy fence—David was forced to tear it down after numerous complaints from indigenous pedestrians.

Frederick's Austrian Castle

Adding to his collection of international homes, Frederick Koch spent a small fortune on his very own castle, Schloss Blühnbach, in 1989. The castle is first mentioned in 1431, where a legend links the site to a miracle preformed by a medieval Austrian archbishop, and the property once belonged to the ill-fated Austrian Archduke Franz Ferdinand. Before Frederick owned the property it was partially open to the public, but since his purchase it's been sealed off entirely.