Under the dominion of the Orsini, in 1455 a private chapel was constructed inside the Casoli castle which, later, with the creation of two naves and a further expansion, became the church of Santa Maria Maggiore. The chapel was located in one of the wings of the castle and together with the castle it forms  the central nucleus of the ancient village of Casoli. A second enlargement can be traced back to the second half of the 1700s, a period in which the choir stalls and the Purgatorial arch that joined the church to the castle were also added. In 1868 the church was further restored and it took on its current appearance. The church forms part of an architectural complex of great interest in the ancient heart of the town. It is connected to the Mascantonio castle through a cross-vaulted portico that leads to the adjacent entrance to the castle. The entrance is accessible via  a set of stairs. The façade is divided into three parts, the central body is separated by two pairs of piers -  square bottomed columns -, these support an entablature  above which is a  triangular tympanum. Above the entrance is a smaller tympanum and a small lunette shaped window, in the two sides another there are two windows. The tower is located on the right of the  church and is a quadrangular shape. The bell tower is created in stone ashlars with other stone set into it .The inside of the basilica has three naves with side chapels and apses. The church houses numerous canvases: a Madonna with Child, San Giacinto and San Biagio by Francesco Maria de Benedictis (1848), San Gilberto by Pasquale Bellonio di Ortona (1797), the Madonna del Carmine and  the Madonna del Rosario (1572) by unknown artists.