Ostland stikes me as Swedish-like in flavor given that the names "Ostland" and "Zeaburg" are (at least phoenetically) similar to place names in Sweden. So, some thoughts based on that assumption...
The Kingdom of Ostland is unique among the other Northern realms, being a nation of islands with no direct connection to the mainland. The largest island hosts the foreboding, grey sea-citadel of Zeaburg and is the seat of the Hoghavkung (roughly the "High King of the Seas"). . The smaller islands form kingdoms unto themselves, and their rulers (the Underkungar) are nominally vassals of the Hoghavkung. The "laws" of the land are maintained strictly through "right of might".
The High King enforces his authority through liberal use of his personal army (the Blodvakt), which are feared throughout the Northlands for their relentless ferocity. The Underkungar also maintain their own standing armies, but in times of war, large units of warriors can be assembled from among the population of all the kingdoms. These troops (known as the Leidangr) are more effective than the typical peasant militia of other realms due to Ostland's prevailing warrior-culture.
The smaller islands are also havens for bands of sea raiders. The activities of these raiders receive little in the way of authoritative attention from the Kings of Ostland, much to the irritation of the other Northern realms. In return for their autonomy, these raiders form the "naval arm" of the Leidangr in times of need.
During the winter months, the harbors of Ostland become ice-choked and passage by sea is treacherous. During these times, Ostlanders keep to their mead halls and quarrel among themselves, but come First-thaw, the raiders resume their depredations. However, also during this time, great fishing fleets issue forth from the islands, trolling the western straights and hunting in the deep waters of the ocean to the east. What little trade Ostland does conduct in the warmer seasons consists of exports of deep-sea fish, whalebone, whale oil, and salt.
While not overly religious, the peoples of Ostland are a sea-dependent culture, and as such, they pay their respects to a great and cruel Sea God, the Great Serpent (Ormungand?). Ritual is based on acceptance of the callous might and dominion of the Sea God; offerings and sacrifices are meant to sate the Cruel One so that he does not bring his wrath to bear on the Ostlanders as they go about their daily lives.