The original structure at Grain Tower, built in 1855, was based on the earlier Martello Towers that were first constructed as a defence against Napoleon in the early 19th century.  It stands off shore on Grain Spit in the Medway and the original tower was built of brick covered in granite.  The fire from Grain Tower would support that from Garrison Point and would defend both the entrance to the River Medway and the sea front of Sheerness.  The guns of the 1855 Tower were mounted on the roof and fired en barbette (that is, the gun is fired over a wall rather than through an embrasure).

In the early 20th century the tower was upgraded to be armed with two 4.7in BLs.  In order to accommodate these guns a raised concrete and stone structure was built on the roof, which in addition to providing a platform for the guns, also provided shelter that could be used for stores and fire control.  At the same time work was undertaken within the main body of the tower to make better ammunition storage to supply these new guns.

Shortly after these modifications, a boom defence was constructed across the River Medway towards Sheerness.  The tower became an anchor point for this boom, connecting it to Grain beach.

Further additions and modifications were made during the Second World War, in particular the large roofed emplacement that supported a twin 6pdr QF gun.  Behind this was a directing tower and a light emplacement.  The biggest addition at this stage was the barrack block; it was made of concrete and stands on stilts with access to and from the main tower.