The bastion is the dominant element of modern fortresses, given that the rest of the fortification is structured around it. Bastions are pentagonal structures that project outward from the main body of the fortification. Their basic function was to cover the blind spots that could be caused by other elements. That is why they all had sentry posts at their most acute angle and cannon embrasures looking out in different directions. Bastions with four sides instead of the standard five are called semi-bastions.