Asserbo Charterhouse is a fortress and Carthusian monastery ruin in the small town of north of Frederiksværk on North Zealand in Denmark. The monastery was founded by Bishop Absalon in the later part of the 12th century and functioned as a short-lived Carthusian monastery. It later came under Sorø Abbey and parts of it under Esrum Abbey and remained so until the end of the middle ages. In that period the buildings were used as a farmhouse or small manor and administered as a so-called Grangier (farmhouse and nearby buildings) by the monks of Sorø Abbey. Originally it comprised a main building, a tower with a cellar, various cellars and some half-timbered houses. The buildings were founded on a structure of large bricks, known as . The site was surrounded by an embankment and access was on