The Martello Tower at Slaughden, to the south of Aldeburgh, is the most northerly of the east coast towers: there were originally 18 in Suffolk. It has an unusual quatrefoil design. The series was constructed between 1808 and 1812 to prevent an invasion during the Napoleonic Wars.
The tower at Shingle Street is a more standard round design.
The tower at Alderton is located to the south of Shingle Street. (Notice the WW2 pill box located to the north.) This gives a view towards the next two towers at Bawdsey and Bawdsey Cliffs.
A single tower guarded the entrance to the Deben at Felixstowe Ferry opposite Bawdsey.
Heritage Futures 
Click to access english-heritage-martello.pdf