To find out what stairs you can currently challenge someone in, first go to the Main Stairs Page, then click on "Show your stairs only" link. The ones in BOLD are ones you can make a challenge in.
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Come to think about it, not instead of... add it as another alternative, since if a player proved a worthy opponent, it's perfectly legit to want to challenge him/her again for another standard game...
(hide) If you don't want other users to know what you are doing you can change into cloak mode in Settings (paying members only). (pauloaguia) (show all tips)