Short answers, pulled from the story.
Roskilde Fjord lies north of the city of Roskilde, Denmark. It is a long branch of the Isefjord, with several cities including Frederiksvaerk, Frederikssund, Jaegerspris, Jyllinge, and Roskilde holding coastline along its shores.
Around 1000 AD, the people of Roskilde deliberately sank a number of their own ships at Skuldelev in the fjord to block Viking raiders from entering. Five ships were initially discovered at the site, and a later expansion of the museum housing the finds uncovered nine more, for a total of fourteen wrecks.
They sank the ships circa 1000 AD as a defensive barrier at Skuldelev to prevent Viking raiders from sailing up the fjord and attacking Roskilde. The sunken hulls were intended to block passage through the waterway.
Fourteen ships have been uncovered at Skuldelev. Five were discovered first, and nine more came to light during an expansion of the museum that houses the finds.
Five cities border Roskilde Fjord: Frederiksvaerk, Frederikssund, Jaegerspris, Jyllinge, and Roskilde, which includes the district of Himmelev. Roskilde is also the home of the Roskilde Festival.
The Roskilde Festival is held in Roskilde, one of the cities with coastline on Roskilde Fjord. The city of Roskilde, including its Himmelev district, sits at the southern end of the fjord.