My iron forge melting contents of large Dorset fortification (5,6)

'fortification' is the definition.
(I know that Corfe Castle is a castle)

'my iron forge melting contents of large dorset' is the wordplay.
I cannot really see how this works, but
'my' could be 'cor' (both are expressions of surprise) and 'cor' is found within the answer.
'iron' could be 'cast' (cast iron is a kind of iron) and 'cast' is found within the answer.
'large' could be 'l' (eg in clothes sizes) and 'l' is located in the answer.
The remaining letters 'fee' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?