In Diamond, Pearl and Platinum below Hearthome city there's that mansion and the trophy garden. At either end of the coridoor there are two doors.
My question is how do you get through those doors?
If someone could tell me, I'd be really gratefull as its been bugging me for nearly a year.
You can't, and there is nothing out there anyway (people have verified using "walk through walls" cheats).
In Platinum on the left side you can battle the 5 maids with Clefairy, but that's it. The doors are not real doors.