St. Petersburg
Kiel There are several notable port cities in Germany that are on or near the Baltic Sea, but Kiel is the largest and most profitable.
Because of the Moscow Canal, the city has access to five seas, the White Sea, the Baltic, the Caspian, the Black Sea and the Sea of Azov
Gdansk is in Poland. It is a port city located on the Baltic coast.
St. Petersburg is a port on the Baltic Sea
A 'baltic port', is any port on the Baltic Sea, which is situated between Scandinavia and northern-central Europe. Oslo, Stockholm, Helsinki, Gdansk, Rostock and Copenhagen are all Baltic ports.
If you mean Tsar Peter the Great, then St Petersburg
Finland's Arctic coastline has no significant navigable port. The closest viable commercial port in the vicinity of the Finnlandic arctic is Murmansk in nearby Russia. This limits Finland's access to the Atlantic to its Baltic Sea coastline.
The Russian Federation has multiple port cities. Vladivostok is their major hub for the Asia-Pacific. Kalinigrad and St. Petersburg allow for ultimate access to the North Atlantic. Murmansk allows for Atlantic and Arctic Sea access additionally. The Mediterranean and Black Sea have port access through Crimea (acceded by Russia in 2014) and Sochi.
He wanted access to the Baltic to sea. St. Petersburg was built after the great northen war for Russia to profit greatly with the new access to the baltic. Many would also belive that Peter also wanted to gain Finland because it was bordered more to Russia then Sweden(do not tell a fin they are more Russian then swedish or you will regret it.). In the end Peter won the land and Russia still occupies it to this day.
Gdansk in Poland
There are a number of major ports that provide access to the Baltic Sea. The only country that is specifically known for having two is Russia which has the port of St. Petersberg in the North (between Finland and Estonia) and Kaliningrad in the South (between Lithuania and Poland) in Kaliningrad Oblast.
Lithunia's main port is Klaipeda, on the south eastern side of Baltic Sea