How does casemate armor works?

[TK3600]

Lets say I have a BB with 10 inch main belt, no casemate. Would the casemate position be unarmored and vulnerable? Most likely not. OK, but what if I add a casemate gun to it, how would armor work? Lets say the casemate has only 5in armor.

 

Possibilities:

1. Enemy shoots the casemate with 6in penetration. Had higher pen than 5in casemate armor, so it count as full penetration. Had casemate not be there weakness would not exist, 10in belt would have blocked it.

2. The same 6in pen manage to destroy casemate, but do not penetrate the hull due to less than 10in.

3. The 6in pen cannot destroy casemate, because casemate armor count as a layered armor behind the belt. 6 is less than 10+5in armor.

Which situation would happen, if the thin casemate is shot?