In Kingdom Hearts 2 how does roxas get in twilight town

It is explained in Kingdom Hearts 358/2 Days.

*SPOILER*

Riku captures Roxas after being defeated. However, he has to tap into the dark power in order to do so. That's why he looks like "Ansem" Riku/Ansem takes Roxas back to Diz and Roxas is place in a simulation Twilight Town while Namine works on Sora's memories, which she messed up in Kingdom Heart: Chain of Memories.