I don't know if Castle Siege is replaceable, haven't tried it myself since I like Castle Siege...
but anyway remember Castle Siege is a multi-pac stage (like Metal Gear, Spear Pillar, etc.)
name the files like this and see what happens...
STGEMBLEM_00.PAC
STGEMBLEM_01.PAC
STGEMBLEM_02.PAC
Hope this helps...
Yeah see if this helps