[Twan]

The problem I see with a (good) castle on the isle is more if the province is very contested (i.e 4 or 5 players owning neighbouring provinces and fighting to be the one taking the isle). In this situation, a siege can be very hard to end, as the competition between other players can help a lot the sieged one (ie : player A siege B, player C, D E try to take the province, C wins but after the 2 or 3 battles he had to fight the surviving army is too small to breach the walls, next turn same situation, etc...). An army with nature mages and good units may resist very well in this kind of situation, especially as spells cast on the province by all players but one will affect the sieging army first (iirc only the player who is sieging can affect defenders with spells, all spells cast by third parties rather affect the bessieger - but I may be wrong).

Of course diplomacy may solve the problem, but as there is only one VP I think it would be hard for all the wanabe bessiegers to find a agreement.