From: Santa Rosa, CA
There are limits to building forts listed below. If units are adjacent to an enemy hex than they should be able to build up to level 3(10%). If you have a situation like this, please attach a save or email it to firstname.lastname@example.org and tell us what hex is not building correctly. Thanks.
18.104.22.168. Limits on Fortification Levels
Requirement to build up to Fortification Level 5: Only will be built in hexes that have a fort unit and a port of at least size 4. Once built, the fort unit is not needed to keep the level 5 fortification. Not possible in a swamp hex.
Requirement to build up to Fortification Level 4: Must have a fort unit in the hex and the hex must either be City/Urban/Heavy Urban terrain or contain a port (of any size). Once built, the fort unit is not needed to keep the level 4 fortification. Not possible in swamp hex.
Requirement to build up to Fortification Level 3: Must either be 1) adjacent to an enemy hex, or 2) a City/Urban/Heavy Urban terrain hex, or 3) contain a port (of any size), or 4) contain a fortified zone unit in the hex. Once the level 3 is reached, the condition does not have to continue to be met to keep the level 3 fortification.
Requirement to build up to Fortification Level 2: Must be a coastal hex (hex adjacent to a major sea, not just a lake).
Requirement to build up to Fortification Level 1: Must be within 20 hexes of an enemy unit. If a hex meets the requirement to build to a given level, it need not meet the requirements of the lower levels in order to build to the lower level (i.e. a hex that qualifies to be built to level 3 only needs to meet the Level 3 requirements in order to build to level 1, 2 or 3). Fortification levels that have reached their maximum fortification level for the hex may continue to build up to 10 percent towards the next fortification level.
All understanding comes after the fact.
-- Soren Kierkegaard