Drusselstein is the fictional Eastern European country in which Dr. Heinz Doofenshmirtz used to live, Heinz was born in the village of Gimmelshtump. It appears to be a parody of countries like Germany, Austria, Hungary, Transylvania (Romania) and Switzerland.
Drusselstein was referred to as "the old country" by Reginald Fletcher, which generally indicates a European country like Hungary, Romania, or going further back, Transylvania. ("The Monster of Phineas-n-Ferbenstein".) In another episode, many people claim Doofenshmirtz had "some kind of a German accent", implying that Drusselstein is in Germany, possibly one of the many states (such as Bavaria, Brandenburg, Saxony-Anhalt or Hessen) However, in "The Doonkelberry Imperative", Drusselstein was shown to be a country of its own, directly across the Atlantic from Danville. However, Drusselstein appears to be on the east coast of the Baltic Sea as there is a route directly to the Atlantic Ocean, assuming that Danville Is in the Eastern United states.
The style of the buildings (Fachwerkhäuser), the food, the dress, and the Carnival (Fasching or possibly Oktoberfest) heavily imply that Drusselstein is somewhere in Bavaria, possibly near Munich. However, Doofenshmirtz's father wears odd viking-like clothing and speaks with a distinctively North German accent (A Bavarian would shorten bewege dich nicht to beweg' dich nicht. It's possible that his father actually hails from the North, Schleswig-Holstein or possibly, Denmark.
The people of Drusselstein live a very rural lifestyle. Children in Gimmelshtump like to play games such as Kick the Schtumpel, and other sports include Poke the Goozim with a Stick, and also baseball, as there is a Drusselstein minor league baseball team. The game that seems most important to the Doofenshmirtz family is kickball. It is especially important to Heinz Doofenshmirtz's mother, who gives all her love to Roger, her youngest son because of his amazing kickball-skills. There are also many skill cranes in Gimmelshtump, which mostly contain rocks. Pastry delicacies include Doonkelberry cake and Doonkleberries. The state still retains figureheads as head of state, including Princess Baldegunde. While most of the time, the people are happy (albeit except for Heinz Doofenshmirtz), poverty still exists. 
At the end of "The Doonkelberry Imperative" the people of Drusselstein created a new holiday; in a honor for solving their central shaft problem, after Phineas and Ferb. They called it "Phineas and Ferb day" or "Ferb and Phineas day", which led the people to fight once again.
The people have a strong belief in black magic. As a tradition, lawn gnomes are placed in front of each house to ward it off. When lawn gnomes become repossessed, parents appear to do anything to make up for it, even making their children dress up as one. Their beliefs are so strong that Heinz had to let his best friend Balloony go because his father told him to stand still.
It is also possible that they believed in ghosts and Princess Baldegunde exclaimed "You are my Doppelgänger!!" as soon as they met. As it is a German country, the state religion is most likely Christianity. It could either be Catholic or Lutheran depending on where in Germany it is (The South is predominantly Catholic, the North/East is mostly Lutheran and the West is a mix of the two).
The people of Drusselstein speak a language that has branched off from German (Deutsch). For instance, when young Doofenshmirtz is instructed not to move in a flashback in "Lawn Gnome Beach Party of Terror", he was told, "Bewege dich nicht!" ("Do not move!"). The words "Das Kind ist ein Schnitzel!" ("The kid is a Schnitzel") and "Zu nah!" ("Too close!") were spoken by a townsfolk and Doofenshmirtz's father during a flashback in "Split Personality".
Otherwise, characters generally speak English, such as the Gunther Goat Cheese's employee who tells the young Heinz to lock up after his party is over ("Raging Bully"), but that was probably not meant as if the populace spoke English, but is rather used for convenience's sake and because it has no direct comedic use as opposed to the former example.
Points of Interest
See also the image gallery for the village of Gimmelshtump.
Department of Motor Vehicles and Goat Registration
Be prepared to wait in line here behind a goat. This is where Doofenshmirtz came to get his driver's license renewed with Perry ("The Doonkelberry Imperative") by doing the dangerous Drusselstein Driving Test Waltz.
Master Drakkenblad's Pretzel Palace
Doofenshmirtz gets a summer internship here but is eventually ruled unworthy and shown the gate ("Buford Confidential").
Drusselstein is powered by belts connected to a horizontal rotating shaft that extends the length of the country. At each end of the huge shaft is a gigantic treadmill upon which goats trod. The kids arrive to a country that has ground to a halt amidst an argument about the direction that the goats should walk ("The Doonkelberry Imperative").
The country has a trebuchet--presumably for defense. As the planet is still distorted by Baljeet when the kids are ready to leave for home, they find the trebuchet a handy conveyance ("The Doonkelberry Imperative").
Zengle's Fruit Stand
The kids travel to Drusselstein to Zengle's to get doonkelberries so that Linda can make a doonkelberry pie ("The Doonkelberry Imperative").
- In the flashback when 2nd Dimension-Doofenshmirtz lost his Choo-Choo, he is a 2nd Dimension Drusselstein.
- Drusselstein literally translates as "stony rock" [from the German, Drusel (small rock or stone or geode) and Stein (stone)]. An appropriate name, judging by the terrain.
- While Drusselstein is said by Ferb to be an Eastern European nation, it apparently borders the Atlantic, making this statement impossible.
- In response to a fan's question about the country's name, Dan Povenmire said he usually spelled it as "Druelselstein", but this was before an official spelling had been selected.
- In Phineas and Ferb: Mission Marvel, Red Skull states to have a cousin who married someone from Drusselstein, and declared that she is dead to him.