Renowned American designer Ralph Lauren has long been known for his love of traditional British style (his redesign of the Wimbledon tennis tournament’s uniforms—which you can buy—somehow made them look even more English), so it was perhaps inevitable that he would end up at the heart of the monarchy, sponsoring a gala dinner in Queen Elizabeth’s favorite residence, Windsor Castle. Hosted by Prince William in the gilded splendor of St. George’s Hall, the occasion marked the announcement of a new Lauren-funded breast cancer research center at the Royal Marsden, the United Kingdom’s leading cancer hospital (of which William is president). Welcoming each guest to the stately locale, the Duke of Cambridge chatted with some of Hollywood and fashion’s biggest stars, including Emma Watson and Kate Moss.

While Prince William’s wife, Kate Middleton, did not attend the event (she was tending to the couple’s adorable son, Prince George), there was no shortage of star power or outstanding fashion moments, most of them involving Lauren’s designs: Watson looked stunning in a crisp white Ralph Lauren Collection shirt and floor-length silk jersey gray skirt with back draping, Cate Blanchett was stately in a Ralph Lauren Collection white columnar gown with diamond chandelier earrings, Margot Robbie dazzled in a gray Ralph Lauren Collection gown, and Benedict Cumberbatch looked dapper wearing a tuxedo by the designer. Other attendees included Cara Delevingne, Downton Abbey’s Lily James, Helena Bonham-Carter, Lauren Bush Lauren, and others from the Lauren clan. Talk about a star-studded affair!

See photos from the event, plus celebrities at more of this week’s hottest parties in our gallery.