Kattegat was home to Ragnar, Lagertha, and their family is most respected in Valhalla, while also being much richer and more influential.
Kattegat spent the six seasons of History's Vikings as the centerpiece of the show's character stories but managed to become more significant in its sequel, Vikings: Valhalla. Set a hundred years after the events involving the legendary Ragnar Lothbrok (Travis Fimmel), his sons, and Maid of Shield Lagertha (Catherine Winnick), Valhalla focuses on telling stories loosely based on the histories of other historical figures such as Greenlander Leif Erickson (Sam Corlett) and his sister Freydís Eiríksdóttir (Friday Gustavsson) and King Harald Sigurdsson (Leo Suter) of Norway. Contrary to real history, where they did not meet each other in the Vikings: Valhalla, Fredis, and Harald meet in Kattegat and become lovers, Harald also gets acquainted with Liv in the important port and later becomes his brother-in-arms.
The town of Kattegat Fjord was a Viking staple, as it was ruled by Ragnar and all the raids he led into England deviated from it. In Vikings: Valhalla, Kattegat arguably becomes even more important, as it is not only in a convenient location to welcome all the Vikings who arrive at the invitation of King Canute of Denmark (Bradley Frigard), but it is also the best place to let the Viking fleet of many nations depart for England. In the hundred years since Ragnar ruled until Caroline Henderson's Jarl Haakon became its current governor of Valhalla, the Kattegat has become larger and more pluralistic.
The main reason Kattegat is so important to both the Vikings and Valhalla has a lot to do with its strategic location. It is a port city and is especially meaningful where Viking heroes thrive as navigators, expanding their fortune and empire through raids or trade, and access to the sea is essential to both institutions. While the Vikings never made clear Kattegat's exact position other than his presence in Norway, the city's innate defensive ability is crucial. The idyllic location of Kattegat is emphasized, as it is secluded by mountain ridges on one side and sheltered by water and fjord on the other. It also appears in Vikings: Valhalla that Kattegat's strategic location as an important port city has placed it on a trade route; Hence, it has many cultures and sellers from all over thriving in its markets. However, Kattegat is the last major stronghold of the pagan Norse religion in Viking culture, a religion that was slowly eradicated by the Scandinavians who converted to Christianity. It's one reason why both Jarl Carr and Olaf Haraldson want to beat her up.
Unlike many of the major sites in Vikings and Valhalla, Kattegat as a city does not exist in the real world, as in fact, it refers to a strait between Denmark and Sweden. During the beginning of the Vikings, the value of the Kattegat was entirely dependent on its location alone, Ragnar, his second wife Aslug (Alisa Sutherland), and the rule of Lagertha, all of which improved the position as a trading center. Lagertha's war against Aslug to take back Kattegat in Vikings Season 4 and the siege of Ivar (Alex Hogg Andersen) on Kattegat led by Lagerta in Season 5 transformed Kattegat into a more important military outpost.
Part of the reason Kattegat exudes strength and importance in Valhalla may be attributed to its convenient location, but the port city's deeper significance must be attributed to Kattegat's place in the hearts of Viking audiences and the shared history of the Vikings and Valhalla. The Vikings series from Episode One makes it clear that they cherish the world created by the Vikings, with many characters referring to the "Great Ragnar Lothbrok" and his exploits throughout the first season. Vikings: Valhalla maybe a Viking sequel that focuses on completely different characters, but admiration for it is reflected in the high regard Kattegat is held by all.