Fonthill, also known as Fonthill Castle, was the home of the American archeologist and tile maker Henry Chapman Mercer, in Doylestown, Pennsylvania.

Fonthill Castle was the home of the archaeologist and tile maker Henry Chapman Mercer. Built between 1908 and 1912, it is an early example of poured-in-place concrete and features 44 rooms, over 200 windows, 18 fireplaces, 10 bathrooms and one powder room. The interior was originally painted in pastel colors, but age and sunlight have all but eradicated any hint of the former hues. One room in the Terrace Pavilion (built on the site of the former home's barn), has a restored paint job so visitors can view the home's former glory. The castle contains built-in furniture and is embellished with decorative tiles, made by Mercer at the height of the Arts and Crafts movement. The castle is filled with an extensive collection of ceramics embedded in the concrete of the house, as well as other artifacts from his world travels, including cuneiform tablets discovered in Mesopotamia dating back to over 2300 BCE. The home also contains around 1,000 prints from Mercer's extensive collection, as well as over six thousand books, almost all of which were annotated by Mercer himself.

The Castle was listed on the National Register of Historic Places in 1972, and was later included in a National Historic Landmark District along with the Moravian Pottery and Tile Works and the Mercer Museum. These three structures are the only poured-in-place concrete structures built by Mercer. The Moravian Pottery and Tile Works is located on the same property as Fonthill Castle, and the Mercer Museum is located about a mile away.

Fonthill Castle and the Mercer Museum are operated by the Bucks County Historical Society, whereas the Moravian Pottery and Tile Works is operated by the County of Bucks.

Fonthill Castle is open Tuesday- Sunday for guided one-hour tours. Due to its historic nature, Fonthill Castle has limited accessibility with steep stairs and narrow, uneven passages throughout the site. Personal photography is permitted during tours; commercial, wedding, or event photography permits are available.

Throughout the year, community programs are held at Fonthill Castle as enrichment activities and/or fundraisers for the Bucks County Historical Society. These events have included Winter Wonderland Tours, Shakespeare in the Park, Candlelight Holiday Tours, Tower Tours for Families, Mercer's Night Tales, Behind the Scenes Tours, and Escape Rooms.

The house is featured (by name and description) in Lewis Shiner's 1990 novel Slam, although in the book it has been relocated to Galveston, Texas.

On the Viva La Bam Seasons 2 & 3 DVD, it is revealed that Bam Margera shot the video for "And Love Said No" by HIM at Fonthill Castle. In 2007, Margera filmed parts of Minghags at the Fonthill Castle.

• Fonthill Abbey – a demolished house in England

• Official website
• Historic American Buildings Survey (HABS) No. PA-1140, "Fonthill, East Court Street, State Route 313 vicinity, Doylestown, Bucks County, PA", 24 photos, 5 data pages, 2 photo caption pages
• HABS No. PA-1140-A, "Fonthill, Garage", 2 photos, 1 photo caption page
• HABS No. PA-1140-B, "Fonthill, Farmhouse", 1 photo, 1 photo caption page
• HABS No. PA-6670, "Mercer House Tiles, Doylestown, Bucks County, PA", 1 photo, 1 photo caption page