Model Lydia Hearst's Epic Bachelorette Bash at Family Castle!

Lydia Hearst's bachelorette party was a perfect fit for her last name -- no cheesy Vegas strip club, instead her turn up went down inside historic Hearst Castle.

Sources close to Lydia -- who's engaged to TV host Chris Hardwick -- say she took 18 friends to the San Simeon, CA landmark built by her great-grandfather ... newspaper magnate William Randolph Hearst.

Her guests included Cara Santana, Michelle Trachtenberg, Clare Grant (Seth Green's wife) and actress Jillian Murray. The ladies were treated to a 2-hour VIP tour ... complete with a swim session in the famed indoor Roman pool (usually off limits). The floaties were pre-approved.

We're told private events at Hearst Castle typically cost up to $22,000 ... but with the family discount Lydia's tab came to $0.00.

Perks of royalty.