Castles were a common sight in the Middle Ages, which lasted from roughly 500 to 1500. Most castles were built to be the fortified residences of local nobility during this politically tumultuous period, but they also functioned as territorial defenses.

Giant stone castles were challenging to take but were not unconquerable. Many fell through sieges, through direct assaults or by bribing the defenders to abandon their posts. But some castles withstood the slings and arrows of their times.

Here are six medieval castles that were never conquered, including one that may have been saved by a dead-cow catapult.

Bohus Fortress (Sweden)

Bohus Fortress, in what is now southwestern Sweden, withstood at least 13 sieges without being taken, according to the Swedish National Property Board. Originally constructed on top of a hill in the 1250s on the orders of the Norwegian king Bohus, it was expanded into a wooden castle in the 14th century. Over the following two centuries, most of the wood was replaced by stone.

At its height, the fortress had multiple towers connected by a complex system of interlocking stone walls. This design meant that if one of the towers was taken, the remaining defenders could continue to fight in another. In 1566, Swedish attackers succeeded in capturing one of the towers. The castle's defenders kept fighting in the other towers and even managed to ignite a gunpowder magazine in the captured tower that then blew up the Swedish invaders.

Bohus was initially built on the old border between Norway and Sweden. But through the peace treaty of Roskilde in 1658, the border was moved and Bohus was ceded to Sweden.

Hochosterwitz Castle (Austria)

Hochosterwitz Castle, in what is now southern Austria, was built on top of a steep hill that rises about 500 feet (150 meters) over the surrounding valley, according to Hochosterwitz Castle's website.

A castle existed on the site at least as early as A.D. 860 and was continually modified and repaired over time, but it does not appear to have been conquered. In the late 16th century, to guard against Turkish invasions, the nobleman who owned the castle built 14 gates, each equipped with different defensive structures.

According to one story, which may be fictional, Hochosterwitz was almost conquered during the 14th century, when the castle was laid siege by an army loyal to Margaret, Countess of Tyrol. The defenders were nearly out of food, so they slaughtered their last cow, filled it with grain and catapulted it at the enemy. Margaret's army concluded that if the defenders could afford to catapult a dead cow with grain in it, they must be well supplied ‪—‬ and they broke off their siege.

Burgdorf Castle (Switzerland)

Burgdorf Castle, near Bern, Switzerland, gave its medieval attackers a lesson in 1383: If you can't conquer it, then buy it.

During a war between the city of Bern and the counts of Neu-Kyburg, the castle, which was controlled by the Kyburgs, was besieged for 45 days by an army sent by Bern. The siege failed, and the Bern army faltered, according to the Historical Dictionary of Switzerland. However, the Kyburgs and the city reached a deal in which the city paid 37,800 guilders, a currency used in the Holy Roman Empire, in exchange for control of the castle.

Burgdorf is far from being the largest medieval castle, but it is sizable: The castle has a residential tower, a keep (fortified tower) and a great hall, all connected by a system of walls, according to the castle's website. It doesn't seem to have been conquered during its 800-year history.

Mont-Saint-Michel (France)

Located on a tidal island in northwestern France, Mont-Saint-Michel is part abbey and part medieval fortress, and it withstood multiple sieges throughout its history. During the Hundred Years' War — a series of conflicts fought between England and France between 1337 and 1453 — Mont-Saint-Michel was besieged by the English multiple times but never fell. The site's location on a tidal island and the strength of its ramparts made the fortress practically impregnable, the fortress Mont-Saint-Michel website says.

In addition to being a powerful fortress, Mont-Saint-Michel was an important abbey. According to legend, the archangel Michael appeared before Bishop Aubert three times in 708 and asked him to build a sanctuary in his honor at this location, the website notes.

Kost Castle (Czech Republic)

Located in the northern Czech Republic, in an area known as the "Bohemian paradise," the Gothic-style Kost Castle was built in the 13th century and expanded over the following two centuries. It contains a series of walls and fortifications, with the "Great White Tower" (the tower's stones have a white color) being the most prominent. It also has a chapel, farm buildings and even a brewery that was built in the 16th century, the Kost Castle website notes.

According to one story, after a failed attempt to take the castle during the 15th century, the leader of the force that attacked it, Jan Žižka, said its walls were as hard as bone. While the story may be fictional, the castle retains the "bone" in its name ("kost" means "bone" in Czech), and it appears to have remained unconquered throughout its history.

Château Pèlerin (Israel)

Château Pèlerin (also known as Atlit Castle) is a Crusader castle located on the northern coast of what is now Israel. Constructed in 1218 by the Knights Templar, its seaside location meant that it could be resupplied by ships, making it more difficult to besiege. The Knights Templar built the fortress as a way to control the coastal route and take back Jerusalem, which fell in a siege in 1187, according to UNESCO.

Within the walls and towers of the fortress were residential areas and a chapel. Château Pèlerin was never conquered, but in 1291, it was evacuated and abandoned after the Kingdom of Jerusalem fell and the Crusaders lost control of most of the Holy Land.