The Fortress At The River, Casbah Ait-Ben-Haddou

Fortifications also extended in depth, with protected batteries for defensive cannonry, to allow them to engage attacking cannon to keep them at a distance and prevent them bearing directly on the vulnerable walls.
The result was star shaped fortifications with tier upon tier of hornworks and bastions, of which Bourtange illustrated to the right is an excellent example. There are also extensive fortifications from this era in the Nordic states and in Britain, the fortifications of Berwick-upon-Tweed and the harbour archipelago of Suomenlinna at Helsinki being fine examples.
• 19th Century
The arrival of explosive shells in the 19th century led to yet another stage in the evolution of fortification. Star forts of the cannon era did not fare well against the effects of high explosive and the intricate arrangements of bastions, flanking batteries and the carefully constructed lines of fire for the defending cannon could be rapidly disrupted by explosive shells.