It took a considerable amount of time to reach their destination.
Since most of the animals and mutated animals near the castle were vanquished—their Life Essence taken by Calidora, she brought Rex to a deep, narrow gorge that winds its intricate path through the craggy terrain.
A ravine called the Red Border.
Crimson lichen stained the ravine's walls, and a meandering of blood-red water flowed through its depths. Perilous stone bridges span the chasm, providing the only means of crossing. It's the border of the capital city of the Vampire Kingdom.
Based on Calidora's description, this is the first line of defense for the capital city.
Looking at the place, Rex nodded in inspiration.