Really cool idea...not sure what details of the story you have peculating - but if they are entering from the sewer into a basement, is it safe to assume that that access point should be discrete? If so, might be cool to have a trap door, false wall, or have that part of the basement separate from the rest of the layout (so that those in the basement are less likely to notice that there is likely a hidden room there - made a quick model of what i mean.)

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Just a thought. Look forward to seeing how this develops