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Using a Red Hinox, you can get on top of the railing next to the Lorule Castle gate. Once you're up there, kill the Hinox. Start walking towards the door at a certain angle, and after a while Link will clip past it and end up in a small room behind the door. The loading zone for the castle isn't there, and there doesn't seem to be anything in that room and the only way to get out is to use Irene's Bell or deathwarp.
Oddly enough, the Lorule Castle doors are interact-able and the "Open" action can be seen briefly when getting close enough to them. Unfortunately, nothing actually happens when you try to open them.