Peak Fitness Challenge website says this about the Devil's Hall trail:
"This is one of the prettiest hikes in Guadalupe Mountains National Park. It's an out and back hike of 2.2 miles each way. After about a mile along the side of a mountain the trail drops into a white limestone stream bed. Keep going and the sides close in on you. Take time to explore the walls of the canyon. After a small scramble you'll come to the narrow notch of Devil's Hall. Spring flowers and fall colors are both exceptional here."
here, and get started!