In the story I'm working, there is a city by the lake, build upon a river delta. It has full fortifications and the landscape advantage to withstand a long siege by its enemies.
The world could be described as medieval with light magic.
The enemy is technologically advanced, but the defender has magic by its side, making the walls invulnerable. (they have expert engineers, the have invented cannon like war-machines(the don't work with gunning powder), but the walls cant break so the aim inside the city).
The attackers are trying to starve out the population of the city, poison them, invoke plagues etc. But it seems the city can overcome all the difficulties and make it through the difficult winter in which no supplies can reach from allies to help them.
So there is our traitor, what can he do to help the attackers win the siege? I'm looking for ideas.
Some more info, about the situation.
The magic the defenders use, needs the wizards to be besides the wall, touching with their palms, some big sacred symbols in the walls. They are protected under a structure.
The city could sustain itself during summer, cause reinforcements with supplies where able to pass through the enemies forces, from the lake. But the winter up here is too cold, and the lake freezes for some months.(An attack form the frozen lake has been attempted but the wizards achieved to sink many of the enemy forces into the lake).
Now the city has approximately 250.000 population(70.000 soldiers, 100 wizards, 50.000 healthy non trained armed civilians). The populations drops its number dramatically through the first months of the winter. The moral of the defenders is really hight, even after the starvation and the deaths.
After the battle in the lake, number of the attacker are approximately 250.000 soldiers with many war machines, that cannot be used successfully in most of the terrain(the delta of the river creates swamps, half of the walls of the city are behind the river).
So, we are in a situation where the attacker has also problem with the extreme cold and the lost battle. The defender is starving out but it seems as he can make it, even if that means loosing half ore more of his population. If the winter passes and the defender stands still, the attacker would retrait. The attackers want to end it now, they are afraid for more casualties during the cold winter.
The traitor is the key on swinging the game, and he is a medium-ranked official of the defenders army.
What I'm asking you is if you can think of some way the traitor, could give an extreme advantage to the attackers to break the siege, but if the defenders know the exact plan of the attackers could easily win again the battle.(let's assume the traitor is double agent)