Bastion Trace Forts and Lines were first used in Italy in the 15th and 16th Century the idea was not only to be able to fire at the enemy approaching the Line but also to defend the Dry Ditch if the enemy managed to get into it.
The reason it's the shape it is,is so there isn't any blind spots in the Ditch.
From the Bastion Flanks you could see into half of Neighbouring Bastion Ditch and along it's face,also you could see and defend the opposite Bastions Flank and the Curtain Wall.
Cannons were placed on the Flank so they could be used as well as Muskets.
Of course not all of the Cumberland Lines use this exact technique but this was the main form used at the beginning of the Lines life prior to alterations.
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