Orthodox monasteries are fortresses of prayer. From the outside they literally look like fortresses, with their high walls, tall windows and, usually, a very small and well-protected door. These outer walls enclose an entire city. At the centre of each monastery is the church, called the 'katholikon', the place where everyone meets. That's what the word means, after all. The katholikon is where the monks gather for prayer and for their services during the day and night.