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...
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greetings
Hohenlohe
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