Well, if we played it out entirely in a tabletop manner, the only impact on the siege would be the disbanding of any units you lost in any of the PvP battles. If you survived relatively intact, you would still have a large enough army to take Zagreb (since in the game you wouldn't have moved). Of course, if you lost a lot of units (or died yourself) the city probably couldn't be taken. If we instead just kept the game going and increased movement rates (not the ideal solution, IMO) you could assault Zagreb and then continue about your merry way.