(^ this is a response for Pokchaki)
that door is in Mosdeep in ruby&sapphire but its in sootopolis in Emerald. its also in FR LG on one of the Islands.
o_o it was answered.
The locked door is/was only opened by special e-reader cards. And it allowed you to battle pretty powerful trainers.
really? i thought u guys starting to talk about something else!