According to the game manual, Cog is a sea vessel and should not be able to enter into river port like Thorn and Novgorod.
But at the beginning of game, player only have a Cog.
And when try to sail this Cog to river port, it is not be restricted.
Why this happened?

i guess you have got the Limited Edition! There the player starts with a "special" cog, which is able to sail on the rivers, like the river cog you can get by university. with the normal edition you starts only with a snaika.

Lebe jeden Tag, als wäre es Dein Letzter! / Live each day as it would be your last!