After housing changed & the apartment buildings were boarded up, basements appeared beneath them. In each basement there was a locked door which transported you randomly to one of about half a dozen strongly coloured seam streets with puzzles in them. When you exited the street you came out into one of the other apartment basements (again randomly).
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See www.glitch-strategy.com/wik... and www.glitch.com/forum/genera... for more details