The Fort covers an area of about , with enough space for 100 men. It was built with red sandstone from the Runcorn quarries. The height of the walls ranges from to , and the towers are high. The Fort originally had a drawbridge, and a Tuscan portal which bore the coat of arms and the words 'Fort Perch Rock'. At one point it was armed with 18 guns, of which 16 were 32-pounders, mounted on platforms. It was nicknamed the 'Little Gibraltar of the Mersey'.