Though the united Kingdom of Prussia (with the capital Berlin) was a member state of the Holy Roman Empire and later the German Confederation, Prussia proper was not a part of Germany (or any other state) but an independent country until in 1871 when the German national state was established under Prussian leadership. Prussia was thereby incorporated into Germany, as its dominant and most powerful state.
After Germany's defeat in WWII, East Prussia's native German-speaking population was forcibly expelled and the area was divided among three countries. The northern part with the capital Konigsberg became the area of Kaliningrad Oblast, while the southern part was incorporated into Poland. A third fraction, the district north of the river Memel (''Memelland'') with the main city Klaipeda (''Memel''), had previously been split from Prussia and incorporated into Lithuania after WWI, and this was done again after WWII. In the aftermath of WWII, all three areas were ethnically cleansed of the their native German speaking populations.
Because the German speaking inhabitants were forced out and all their property was confiscated, the population of Kaliningrad Oblast consists now mostly of Russian speaking people, but there are still a lot of traces of old German culture which, along with the presence of modern Russian culture, makes it an interesting destination for travelers.
Kaliningrad Oblast produces 90% of the world's amber supply.
The Russian language is spoken by more than 95% of Kaliningrad Oblast population. There is a small Lithuanian-speaking minority in the north-east. English in understood by many people under 25. Many people understand some German and Polish.
There are several border crossing points. From the Polish direction: Mamonovo, Bagrationovsk and Gusev. From the Lithuanian direction: Sovyetsk and Morskoye (on the Curonian Spit). You will need Russian visa. Even if you have one it may take days to get in by car. It is probably quicker to fly to Moscow and then fly or get a train to Kaliningrad.
Kaliningrad Oblast has a small international airport, so you may need to fly into Lithuania (Vilnius), Poland (Gdansk or Szczytno) or Finland and take a bus or ferry to Kaliningrad. -Local KD-Avia is flying to several European cities, this is the easiest way to get into Kaliningrad. You can take a LOT (Polish Airlines) flight through Warsaw, every day except Saturday. Aeroflot also has several daily flights from Moscow.
Curonian lagoon, Vistula lagoon