Why did the DtL need to open?

[gelandporn]

Mickey managed to leave it using just a Card so.... I don't think it's -that- special.

The card guided him, but ultimately:

It seems my friend, fighting in the realm of darkness, appeared in Castle Oblivion through a corridor of darkness constructed by Organization XIII.
My new ally Riku also effected his return via one of these corridors.

 

[HeartSeams]

The card guided him, but ultimately:

Doesn't really make sense since the Org said themselves that they didn't know they were in the castle at first =/

Not to mention, by the time Mickey ACTUALLY gets into CO, all of the Org is dead except for Axel.

 

[skyfoxx]

Doesn't really make sense since the Org said themselves that they didn't know they were in the castle at first =/

Not to mention, by the time Mickey ACTUALLY gets into CO, all of the Org is dead except for Axel.

Well Axel was a traitor to the Organization so...

 

[gelandporn]

Doesn't really make sense since the Org said themselves that they didn't know they were in the castle at first =/

Not to mention, by the time Mickey ACTUALLY gets into CO, all of the Org is dead except for Axel.

I don't know why you're arguing this when it's explicitly stated in the SAR.
No one has to be around for a corridor of darkness to stay open. In fact, as AtW points out, Corridors of Darkness are natural phenomenon and can pop up randomly in the realm between. The word "constructed" heavily implies that this was a corridor which either remained open or could be accessed via using special requirements (eg having that card).