Fortification level

The fortification level of a province or a holding indicates how much additional fortification has been added to the area, above and beyond what would normally be expected. Fortifications are built using the build domain action.
The level of a fortified holding cannot exceed the level of the holding it protects. The fortification only protects holding levels equal to its rating, any holding level which exceed the fortification are subject to destruction.
A province fortification can be built up to level 10, regardless of the province's level. The fortification level defines how many holdings it can protect, according to their holding levels.
Fortifications have a maintenance cost equal to a holding/province of the same level. Thus, a fully fortified holding/province has double the maintenance cost of a normal (unfortified) holding.
Source holdings cannot be fortified so do not have a fortification level.