That's interesting, it's the only inch thick plate I've found, I can't say I've ever fired ball ammunition against steel, but I am surprised it will penetrate approximately half way through one inch thick steel?
I assumed standard ball would simply put a small dish into thick steel plate.
The rectangular building that once stood in front (only the foundations left now) must have had some sort of protection built in. Either that, or sandbagged very well indeed!
Just the armourers hut left standing today.