In the time of Peter the Great, the sea floor just north of the Petergof site and to the east toward St. Petersburg was too shallow for commercial ships or warships. But to the west of Petergof, the sea floor dropped off to be deep enough for sea vessels. Accordingly, when Peter the Great decided to build St. Petersburg at the eastern end of the Gulf of Finland, he built the commercial harbor for St. Petersburg and the Kronstadt fort across the 20 kilometers (12 miles) of shallow sea to supply and defend his Navy.