Ypati was fortified for the first time in Hellenistic times. Parts of this fortification have been found in many places around the Castle and the modern settlement. The fortification was renewed in the Early Christian era by Emperor Justinian (527-565 AD). In 869 at an ecclesiastical meeting for the first time Ypati was referred to and was known under the name of New Patras together with Lamia, which was then called Zitouni. In 1204 he was captured by the Frankish crusaders and in 1218 he became the head of the Byzantine despots of Epirus. In 1267 it was bequeathed to Ioannis Aggelos Doukas Komninos of the north east, who fortified the Castle and made the High Court seat of the state of Great Vlachia, which extended to western Macedonia.

In 1275 Ypati Castle came under assault by Ioannis Palaiologos, a Byzantine general, head of 30,000 men. Its strong fortification prevented its occupation by being raided. Ioannis escaped and summoned for help from the Frank, Duke of Athens, who arrived in New Patras, chief of knights and dissolved the siege. The Castle remained in the dominion of Ioanni's descendants until 1318, when he passed into the hands of the Catalans, who formed the duchy of Athens and of New Patras. In 1393 he was captured by the Turks, led by Sultan Vajiazet. During the period of Ottoman rule, it was called Patratziki (small Patras) and it was the seat of a particular province.

Several remnants of this mighty fortress fortified vigorously and also by nature, survive today. In particular are the ruins of a round tower, unknown from when it dates back to, it is also questionable whether it was constructed by Greeks, Franks or Catalans.