The period when small independent beyliks (principalities) were founded in Anatolia in the 13th century is called the principalities period. The process of creating a new architectural style in Western Anatolia, as well as the works-built marks an important milestone in this period, which is defined as a transition period between the Seljuk architecture and the Ottoman architecture. The Aydın Dynasty, one of the important principalities of western Anatolia, had its capital at Birgi from 1308 to 1426. The dynasty is known for with its economic and political power and the important works it left behind. Helvacılar Mosque is one of the important works of the Aydın Dynasty, which was built in the 15th century and survives today. It is also an outstanding example to understand the single-domed mosque typology in western Anatolia. Helvacılar Mosque, located in the Kuşcular Neighborhood of the Urla District, is today derelict and abandoned. Heavy structural damage in the structure poses a grave risk for the structure to survive. This study compared and discovered the similarities and differences between Helvacılar Mosque and the similar-period structures situated in Urla and identified the architectural and structural characteristics of the structure with the aid of measured drawing projects. This study used a combination of architectural restoration and structural engineering. For Helvacılar Mosque, the study carried out the self-weight analysis, modal analysis, settlement analysis, response spectrum analysis, time history analysis using three real earthquake records in a finite element model. The aim of engineering research is to identify the present causes of damages in the structure. With the help of finite element analysis, the study put forward the essential repair and strengthening methods for Helvacılar Mosque to repair the damages observed in the structure, strengthen the structure, and obviate the causes of the damages based on the intervention methods offered in the guideline titled Earthquake Risk Management of Historical Structures issued by the General Directorate of Foundations.