Oslo's name should be Áslo, and it should have this seal, which is from 1300, the oldest seal of Oslo. (same as the modern seal but instead of a woman lying at the bottom, it should be an armed warrior)
Bergen can still be called Bergen, as you can see on the seal below (Bergensi). Bergen's old seal from the 1290s had two sides, but you can use the castle, as that was probably the most common.
Skara can just be called Skara, here is the old seal.
I can't find any old names for Sigtuna, so just go with that. You can use the same seal, considering it has barely changed over the years, except for the crown.
Kalmar's name should be Calmar, and here is their oldest seal, from the 1250s.
Visby should be called Wysby, here's the seal.
I can't find any old names or seals for Lund, sorry.
Roskilde should probably be called Roskilde. Here is the oldest seal I could find, but it's a bit broken.
Ribe is Ribe, and their seal seems to be the same as it is today.