Saint Michael's Cave is a series, or network of caves made of limestone, which are found on the Rock of Gibraltar. St Michaels Cave is located on what is called the Upper Rock, inside the Upper Rock Nature Reserve of Gibraltar.
The cave was created by the slow seepage of rainwater through the rock, which turned into a carbonic acid solution that actually dissolved the rocks of the cave. The process made the tiny cracks of the geological faults of Gibraltar grow into very long passages and deep caverns over the thousands of years of its formation. The Cathedral Cave, part of St Michaels cave was at one time thought to be bottomless, and was long spoken of in the legends of Gibraltar.
The Rock of Gibraltar has long been considered to be one of the pillars of Hercules, and this too adds to the mystique and legend, and since it hosted the cave, the caverns themselves were thought to be the Gates to Hades, or Hell, an entryway to the Underworld where the dead rested.
In the latter part of 1974, proof that the cave was known to and used by prehistoric men was made clear with the finding of art on the cave walls, showing an ibex drawn there that was traced to the Solutrean period (dating the cave art to about 15-20 thousand years ago), but later, two Neanderthal skulls that were found in Gibraltar tell us that this cave could have been discovered and used as early as 40,000 BC.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.