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The RoA fortresses can hold characters, but never mention when you can put them in/remove them, so they default to the prep phase.
For most fortresses, where you can add/remove characters more or less at will, you're restricted to the prep phase. The RoA fortresses can hold characters, but never mention when you can put them in/remove them, so they default to the prep phase. I don't think Tartaros is subject to that, though.