    |Sentence 1 It is housed in a Martello  IA series of such towers, some 12 m  4@ Ft) high and 25 m (8 ft) thick,  lwere constructed along the coast  lat the beginning of the 19th century  Ito guard against invasion by  Napoleon.    |Sentence 2: A number of these  towers, which were built to guard  gainst invasion by Napoleon, were  uilt along the coast in the 19th  century.                        