The setup is this:
Field spell: Defense Zone
Column 1: Any trap monster / Field Barrier
Column 2: Any trap monster / Imperial Custom
Column 3: EMPTY / Reserved zone for trap monster
Column 4: EMPTY / Reserved zone for trap monster
Column 5: (Anything, preferably something that can prevent battle damage)
The logic goes that Field Barrier protects Defense Zone, The trap monsters combined with Defense Zone protect themselves AND Field Barrier / Imperial Custom from destruction and Imperial Custom protects the monsters from battle.
Save for field-wide bounce or sending, it looks like a perfect lock, but is it rules-legal?