Replay: If you have the code, just pick up decoder strips and skip all way to unlock treasure room safe - the code will be in chat box. If you didn't save the strips and can't remember the combination, follow the steps.
Enter the red barrier directly west of the armoured zombie room.
Head west, passing through 2 additional red barriers.
Inspect the tin soldier on the operating table in the south-western room.
Exit the castle.
Talk to the Trial Announcer at the east Varrock bank to get tin soldier head. (3•1•3)…
Can I have a tin soldier head?
I'd like to get ahead of competition.
I have to go.
Get 3 tin ores, 4 fire runes, and 1 nature rune.
Return to the tin soldier.
Inspect him thrice to activate him. (1)…
Use a vial of red mist on the tin soldier if it has not automatically been applied.