The structure of defenses changed radically between the 14th and 18th centuries as cannons gained more widespread adoption as siege weapons.

Cannons more or less made the previous defensive structure of high stone walls, such as those surrounding castles or walled cities, completely useless. A new defensive structure had to evolve. One of the more successful structures was called the battlement. However, the lower walls of battlements and the longer range nature of cannon sieges left the defender at a disadvantage to covering the battlefield defensively by creating a number of blind spots:

                 Blind                        Blind
                  Spot                        Spot
                      \       o       o      /
                          |   I       I   |
                         o==| -       - |==o
                          |               |
                          |               |
                          |               |
                         o==| _       _ |==o
                          |   I       I   |
                              o       o
                      /                     \
                 Blind                       Blind
                  Spot                       Spot

 |==o is a cannon.

The Italians created the following geometrically-based solution:

               o / \ o           ^
                \   /         o / \ o
               /     \         \   /
               \    ==o       /     \
                /   /-__    o==     /
               o \      --__  \    \
                 /          --_\  / o
              o /                 |
               \                  |
              / /               |==o
             .                    .
            .                     .
           .                      .

This design reduced or eliminated the effective blind spots on the battlefield by increasing the number of sides the defensive structure had. The term "Trace Italienne" is French for "Italian Shape (outline)," as it was the French who first encountered this structure on the battlefields of northern Italy.

This shape proved to be very successful, indeed many American forts display this design. Even the American Pentagon Building exhibits ideas from this enlightened design.

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