Koca Mustafa Pasha Mosque began as a Byzantine monastery dedicated to Saint Andrew and was later converted into a mosque. The site has a long religious history, with early medieval foundations, later Byzantine rebuilding, and Ottoman additions layered into the present complex.
It is well away from the most crowded parts of Sultanahmet, which makes it a quieter stop for visitors interested in Istanbul's Byzantine-to-Ottoman religious architecture. The appeal is in the layered history rather than lavish decoration.