Ruins are named map locations which come in two varieties. Both follow the naming convention "Ruins of <NAME>". Ruins can spawn world bosses.
The first kind is non-linear and appear as groups of several white Greco-Roman structures randomly placed around the ruin location. This type contains Zombies Skeletons, and Frighteners. The jungle variation follows a different architectural style but functions the same way. This type is home to Spike Creatures and Djinns
The other kind of ruins are dungeons which are functionally identical to Castles, and are simmilar in appearance save for large cracks and other damage to the exterior.