Note to Hamlet, 1.1.89: "Which he stood seized of"
The phrase, "all those his lands / Which he stood seized of," may seem to mean that when King Fortinbras lost his life to King Hamlet, he also lost all of his lands, including Norway, to King Hamlet. However, that doesn't seem to be the case. In the next scene, King Claudius sends ambassadors to the King of Norway, so Norway is an independent state, not a Danish possession. Perhaps what is implied is that King Fortinbras and King Hamlet had been fighting over territories outside the borders of either country. Such a territory, "a little patch of ground," held by the King of Poland, is mentioned later in the play.