Complex sentence: Origins of the fort extend far into antiquity but the existing base structure was built during the reign of Mughal Emperor Akbar between 1556--1605 and was regularly upgraded by subsequent Mughal, Sikh and British rulers.
Split into simpler sentences: Origins of the fort extend far into antiquity but the existing base structure was built during the reign of Mughal Emperor Akbar between 1556--1605. It was regularly upgraded by subsequent Mughal rulers and after the fall of the Mughal Empire it fell to Sikhs and British rulers.