How do I build forts?

[Husaria]

Page 55 of the game manual states that to build a fortification I need four artillery "batteries" and two supply units. I cannot seem to be able to build a fort.

What type of arty must be used? Do all four need to be the same? Is it two or four supply units? Should all six units be in the same stack?

What is the resulting "fortification"? Is it a level 1 fort?

Is there any special consideration for the area I am attempting to build the fort in?

What am I doing wrong?

[czert2]

to build supply depot you need 8 elements of supply units (booth supply and ammo/magazine works) - so since normal unit have only 2 elements, you need 4 of them.
for building of forts, you need same supply demans as for depots + arty units, and since i didnt builded forts so much in this game or previous ones, so i can be wrong of this, but 4 elements (or 8 not sure) of arty shoudl be enough (but they must be independent, not parts of other units), and im completly sure that light arty will not work, and not sure about others - field, medium, heavy. mayby field arty will work, mayby not.
and yes, all these unist have to be in same stack to be able to build fort, and no others units are needed.
so it is good way to find which arty units will work and which not.

result of this shoudl be fortified line/trenches lvl 1 (from different game), result of fort/redoubt lvl 1 is unlinkely since booth these requred of laabor work to build them (many ground moving, pouring concerte..etc).

and yes, you can build it on any area (well mayby except one which allready have betetr fortifications), but due to cost involved it is best to build it at important area (cities you want to defend, fronts you want to hold at all cost, very ideal defensive teran to hold advancing enemy..etc).