The exterior of the simple building belies its lavish interior. The chapel is built without a division between chancel and nave and is very Catholic in its decoration scheme - not too surprising when you consider that Wynn consulted with a Jesuit priest over part of the design.
The interior retains much of its 17th-century character, with wooden box pews and pulpit adorned with carved figures.
The most interesting feature is the ceiling, painted as a celestial firmament, with angels, doves, cherubs, and other allegorical symbols mixed amid symbols of the sun, moon, and stars. There are carved and painted wooden cherubs attached to the wall panelling. A royal coat of arms of Charles II is set upon the south wall.
The chapel continued to hold regular services until the 1920s, and it is now in the care of Cadw.
Note
As of this writing, Gwydir Uchaf Chapel is open only by arrangement with the keyholders - see the Cadw website for current details.
Update
When we visited, we discovered that you can get the key from nearby Gwydir Castle without pre-arranging a visit. It is only a 5-minute walk from the castle to the chapel, or a very quick car ride if you don't fancy walking up the hill to the chapel.
If you are visiting the castle anyway, I highly recommend a side trip to the chapel; the ceiling is quite wonderful, and the 17th-century woodwork makes the chapel a delight to visit.