Complex sentence: Between 1849 and 1857, two Martello towers of dressed Portland stone were constructed at the south-western and north-western corner of the dockyard; both were garrisoned by sergeants of artillery and their families.
Split into simpler sentences: Between 1849 and 1857, two Martello towers of dressed Portland stone were constructed at the south-western and north-western corner of the dockyard. Both were garrisoned by sergeants of artillery and their families.