The reason for being a World Heritage site is that the construction of these monasteries served as an architectural and urban planning model for the monasteries and towns that followed. These monasteries were built to be very solid with thick walls and had a very austere aspect. In some of the complexes, one can see stone merlons which make the complexes look like castles or forts. These were for defensive purposes, as the monks were invading Mesoamerican lands to impose a new religion. For this reason, churches and monasteries of this type are called “fortress temples.”