[Ozymandias]

When your enemy builds an unassailable stronghold you should fight him elsewhere.

Consider your reasons for wanting to take the extremely well defended province. Three that occur to me might be that you need it to access your enemy's provinces, you need to prevent troop production from that province, or for victory point reasons.

When access to your enemies provinces is restricted to a single province it is a good time to consider diplomacy. Your border must be short, so a prudent garrison need not be terribly large. Furthermore your foe has already made a sizable investment in static defenses which I would not interpret as an aggressive act.

In the case that you need to stop troop production, consider the rituals which cause unrest. If you keep the unrest over 100, no troops will be produced in the castle.

If you must take the province to prevent your enemy's ascension try using assassins and commander slaying rituals on the province. Horrors, Lamashtas, and battlefield wide negative spells are good ways to wreck emplaced forces without using your own armies. Damage shields and morale effects are both good ways to disperse unscripted chaff. Instant travel spells might still allow you to make attacks on other more weakly defended vp provinces. This is also another time to use diplomacy to your advantage, as every other player also shares the incentive to not lose.