There're clearly structures in the North-West portion of The Tower and there's a massive door that alludes to it as well (with a mech standing around that'll no-doubt allow you to trigger the map when it's unlocked).
Care to speculate about what it will be?
Something to do with the factions?
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Maybe some part of the actual City we protect? At least, that's what I hope. You can sort of see the tops of buildings that don't resemble the architecture found inside the Tower looming over the large iron door you're talking about. They also appear to have moss or something growing on them.
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I'm not sure, so don't kill me if I'm wrong. It might be the iron banner event, and it turns color when it opens up.