This natural surface trail winds under the natural bridge and through a large sinkhole, the Colosseum, and is part of the Ha Ha Tonka Karst Natural Area. In spring, you will be treated to a lovely view of flowering dogwood, redbud and serviceberry from the wooden platform where this trail begins. The natural bridge is a massive stone arch left behind when the cave system around it collapsed. It spans 60 feet, is 100 feet high, and is 70 feet wide. The natural bridge was used as a route to the castle until it was closed to vehicular traffic in 1980 because it was too narrow. It is an impressive geological feature and one of the highlights of the park. After a heavy rain, water often cascades off the back side making its way to the lowest point in the sinkhole beyond.
The Colosseum Sinkhole is 150 feet deep. Spring wildflowers are abundant in this sheltered enclave, especially in the section just beyond the natural bridge where the trail is immediately adjacent to one sheer bluff side. As you start your hike out of the sinkhole at the south end, you will step between large boulders evidence of the rocky ceiling that one covered this cave room. It is a long, steep climb out of the sinkhole to near the top of the ridge, but the views of the old post office, spring and castle ruins at the top are a great reason to stop to catch your breath. The figure-eight loop then takes you across the top of the natural bridge and back to the trailhead.