RPG Insanity

First. I sorry for my english, it very bad

Ok, i kill the world tree and i can't go this place ?  Does anyone know how to go in there ?

I can't tell from the photo, but if it is the Next Castle, you need the Key of Next, which can be found in the Mirror Castle.

You should check the post in the forum at Treasure/Item Location which includes several keys and their locations.