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The atmosphere is very different. The megamart is part of Acerola's trial site, so is basically "safe". It's a run down building, but they let you in as part of a structured test so you have someone watching over you.
The Old Chateau is completely optional. First you need to use Cut to access the building. The local Gym Leader (Gardenia) is too scared to check it out herself and gets nervous talking about it. Nobody knows about its history, and there's secret rare Pokemon inside like Rotom or the 1% Gengar. There's also some weird "ghost" NPCs floating around the area if you pay attention.
I think is a good example of how the games have changed desu.
