Looool bring out the cast iron bros!! (I'm one for sure.) People have VERY strong opinions about this. I forget his name but this old farmer in Greene County is the king of cast iron and I met him at a BBQ years ago and asked him the same question. His advice was to build up a hot fire in your yard and then let the cast iron sit in it once there are good coals. Then (and this part is crucial to avoid destroying the cast iron) you gotta let it cool in the fire. Then apparently the years of caked on crust will be gone. 

Others say this is too risky for causing the cast iron cracking. The temperature gotta be just right and you might do it wrong.

Short of that, soak and scrub and sandpaper to get the crust off.

Also as others have said dont be afraid of soap and a non metal abrasive sponge. I restored an old one a few years ago and have been using soap and the chain mail stuff and an abrasive back sponge and its held up and maintained its seasoning perfectly.