The interior was originally painted in pastel colors, but age and sunlight have all but eradicated any hint of the former hues.

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.

[4] Due to its historic nature, Fonthill Castle has limited accessibility with steep stairs and narrow, uneven passages throughout the site.

[4] Throughout the year, community programs are held at Fonthill Castle as enrichment activities and/or fundraisers for the Bucks County Historical Society.