AstralLein
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Do you really know how the Fortress works? First, you upgrade it to the first tier, and then you get the option to upgrade it either to a standard alternative or to a new one.
If you don't get the option to upgrade to a new alternative, it might be because your map isn't patched to EWA.
I don't remember maps for EWA needing to be patched. It changes the game itself, and upgrades are just one of the characteristics, along with other things like speed/attack rate, etc., of the creatures.