Quote Originally Posted by ZIM!!
how do you get an army to Fortify?
The army must have a faction general in it. It also costs 500 dinarii.

The process is almost the same as hiring mercenaries.

Select the general's army. When the round knob showing construction status in the lower right-hand corner is not faded out, you can build a fort. (Just like when the recruitment knob is full-colored, you can hire mercs.)

I think you also must have a minimum number of troops, but I don't know what that minimum is. Also, some terrain won't allow forts.