Friedrichsburg Gate is the only surviving element of the fortress (later a fort) in Königsberg (now Kaliningrad) Friedrichsburg, which is now one of the city's landmarks.

The construction of the fortress was preceded by a rather turbulent period in the city's life: in the first third of the 17th century, the territory of East Prussia became a battlefield between Sweden and Poland, which was a direct violation of the country's sovereignty. Therefore, starting in 1626, the inhabitants of the trading towns near the castle surrounded their territories with a defensive rampart and a water-filled moat. This was done according to a project by Johann Strauss, a professor of mathematics at Königsberg University (namesake of the "Waltz King").

By the mid-17th century, around the three towns (Altstadt, Kneiphof, and Löbenicht), 26 bastions, 8 half-bastions, and 9 main gates had been built. What was missing was a fortress. The Königsberg castle-fortress, founded by the Teutonic Order of Crusaders, had become outdated as a defensive structure in the era of firearms.

They again turned to the university scholars: the best fortress project was presented by court architect and mathematician Christian Otter, who developed the Dutch system of fortress construction — surrounding the fortress with a wide water moat. Christian Otter also proposed the location for the fortress at the part of the defensive rampart where it is crossed by a river. The architect explained his decision as follows: "neighbors will not enter without knocking on the door first, and rebels will not take advantage of the fortifications." Elector Friedrich Wilhelm, who once declared that "the significance of a state without a past, with only a future, must be based on the sword and science," sacrificed his residence for this construction, which was located exactly where the future fortress would stand. "Better to lose the residence than the princely throne," said Friedrich Wilhelm.

The fortress designed by Christian Otter had a square shape on the topographic plan. At its corners were four bastions with melodious names — Emerald, Pearl, Ruby, and Diamond. The names of these stones represent the three medieval towns Kneiphof, Löbenicht, Altstadt, and the Königsberg castle. The colors of the three stones correspond to the coats of arms, and the last one symbolizes monarchical power.

Inside the citadel courtyard were the commandant's office, barracks, Zeughaus (military armory for weapons or ammunition), granaries, guardhouse, prison, and church.

In 1697, the fortress was visited by Russian Tsar Peter I, who studied military sciences at Friedrichsburg and Pillau fortresses under the name of the orderly Peter Mikhailov. Upon returning to Moscow, Peter I received a certificate stating: "Peter Mikhailov is recognized and honored as accomplished in bomb throwing, cautious and skillful in firearms." After returning to Russia, by Peter's order, several forts (Kronshlot, Citadel, and Alexander's Redoubt) were built on Kotlin Island, modeled after the Friedrichsburg fortress.

The Friedrichsburg citadel never engaged in combat. However, near the fortress, Immanuel Kant took his daily afternoon walks, later named the "Philosopher's Walk." In the mid-19th century, during the construction of new fortifications around Königsberg, Friedrichsburg fortress was rebuilt into a fort. In 1852, brick gates were erected, which have survived to this day. The project of the Friedrichsburg gates was by August Stüler, founder of the Berlin Architects' Association and court architect of Prussian King Friedrich Wilhelm IV.

At the beginning of the 20th century, this type of fortification was already considered unsuitable for defense, so in 1910 the German Empire government transferred the Friedrichsburg fort to railway administration. The railway treasury paid the military department compensation of 8.5 million marks. The ramparts were dismantled, and the moats of Friedrichsburg fort were filled in. Railway tracks were laid over the former fort territory, and a freight station was established. Only the gates and barracks of the fort have survived to this day.

The gates of Friedrichsburg fortress, also known as Friedrichsburg Gates, are the only surviving element of the former fortress. These gates were built in 1852 according to Stüler's design. Architecturally, they resemble the city gates of Königsberg. Like the city gates, the fortress gates were built from fired shaped bricks in the Neo-Gothic style.

The gates consist of a central carriage portal in the form of an arch, flanked symmetrically by casemates for the garrison. On the sides of the passage are four round towers. The gates are decorated with ornamental battlements and false Gothic windows.

The Friedrichsburg gates were heavily damaged during World War II and were under threat of demolition (for some time, a field military printing house was located in the gates, then a motor convoy), until in 1960, by decree of the Council of Ministers of the RSFSR, they were granted the status of an architectural and urban planning monument. Despite this status, the gates remained effectively neglected and continued to deteriorate. They were saved from dismantling by being located on the closed territory of a freight railway station. Restoration of the gates began only in the early 21st century. In 2002, the gates received the status of a federal cultural heritage site, and in March 2007, they were transferred to the Museum of the World Ocean.

Sources:

https://www.culture.ru/institutes/1525/fridrikhsburgskie-vorota-v-kaliningrade

https://ru.wikipedia.org/wiki/Фридрихсбургские_ворота