I don't understand this. Why Master Eraqus wanted to transform the destroyed  land of departure in Castle Oblivion and eveyone who get there forgets everything ? To protect it or what ? And if so from who to protect it ?

Yes Master Eraqus wanted to make The Land Of Departure turn into Caste Oblivion to protect it. Also people who go there don't forget everything, Sora only started forgetting stuff because of Namine. 

Castle Oblivion did not cause anyone to forget anything that was Namine. Also the LoD can be transformed by it's protector (Aqua in this case) in order to protect it from those would abuse it's properties. Aqua saw this trick as a perfect way to keep Ven safe because she knew she would be the only one to safely navigate the newly transformed state. She created Castle Oblivion in order to hide Ven while she went to find Terra.

Edited January 2, 201312 yr by devereauxr

Master Eraqus said that the world is unique in the sense that light and darkness exist in balance there. They therefor inveted a trick as he calls it to prevent others from taking advantage of the worlds unique abilites.

Also, everyone who comes there wouldn't forget everything, they would simply be lost in oblivion. The memory loss part was all Naminé's doing.

Yeah what they said...It was Namine but again she was lonely and wanted friendship but then she was thearten by org 13. Castle Oblivion is basically a maze and Aqua I guess basically has the map in her brain..

Pretty much.

 

CO without Namine is just one big maze and only Aqua can navigate it.

 

They don't lose their memories they just get lost.

I'm pretty with Sora, Namine was the one playing with memories.

On top of what everyone else had said, creating the CO also creates the Chamber of Waking. The Keyhole is now sealed away in the CoW making CO virtually indestructible.

They only lose all of their memories if Namine takes them away. Changing it to CO was to protect Ven.