I wish and hope for the possibility of building fortified camps ingame like the Pirna camp because historically there are some decent combats around fortified camps between the Prussian and Austrians in which one or the other side tried to assault these fortified enemy positions.
It even looks like a necessarity for the game because you need some good depots between certain cities to supply your armies in the field and without some fortifications any depot someone builds offers a good opportunity to the enemy to break the supply line and communication very easily. With some decent fortifications like a fortified camp this would become a better challenge for any good PBEM game not only in Campaigns against the AI.
So I hope you can implement this option, but consider that the recruiting costs for the requisite units are very high, so that I suggest that you need only a supply train and an artillery unit each as a pre-requisite building a camp. Perhaps I would consider an upgrade to a fort with doubled costs.
But for a better playability the fortified camp seems to be enough. A Fort upgrade could be an additional option. I even suggest that you should need a Pioneer unit at the Fort location building the Fort or even the Camp.
For the Camp you should need only 15 days, but for the Fort upgrade I consider about 75-90 days as a necessarity because it would be built from wood and some stone materials and therefore you could add some war supply and money as additional pre-requisition. Additionally a camp could be erased because of fort level one, but after upgrading to a real Fort it should be at Fort level 2 and could be never erased anymore as I like to suggest.
A fortified camp in connection with a depot would improve the supply for the field armies and could even serve as a decent winter quartier for a corps at depot level one and for an army at depot level 2-3.
Sofar my opinion and hopes and wishes...
greetings
Hohenlohe