the Greek ships. In the 19th century there was a probability for Hydra to use its cannons again, as the Greeks were at war with the Turks.
Fortunately, the Greek fleet had always managed to protect Hydra, and as a result, the cannons have not been often in use. The bastions with cannons seemed to be a necessary precaution in case Hydra actually got attacked. You can still see them today on the left and right side of the Harbor.