Ajdovščina is one of the best-researched Roman sites in the Vipava Valley, uniquely preserving a nearly complete Roman-era wall with fourteen towers. In the early centuries BC, the Romans built a postal and freight station here, mansio Fluvio Frigido, which later became a fortified military post called Castra. In 394 AD, the Battle of the Frigidus occurred nearby between Emperor Theodosius and the usurper Eugenius, which has lost the battle, paving the way for Christianity becoming the official religion. For a very long time Ajdovščina developed only within the remains of Castra, which was inhabited until the early 5th century, and by the 9th century, it was permanently settled by Slavs.