When you go down the ladder you will land in a fire field
Here you will face 2 vampires, a demon skeleton, and a ghost at the same time. Rarely you can also face Captain Jones. If you consider yourself being unable to face them at once, you are recommended to shoot a destroy field rune on the fire field and rope the creatures one by one.
After clearing all of the creatures in this room, "use" the head of the coffin against the far wall to recieve your Plate Armor.