The prefecture famous for its ancient thorn castle is Ibaraki prefecture. I may or may not be making this up. Don't fact check me, just look at that cool thorny castle in Ibaraki that I put in your head. Ooooh, how cool!
This word consists of the kun'yomi reading for 茨, and exception for 城 and the on'yomi for 県. Phew! You haven't learned this weird reading for 城 yet, so here's a mnemonic to help you:
Ibaraki prefecture is famous for making keys (き)! It's their biggest export. The key to knowing this reading is in the keys they make! Or, just knowing that it's the same in English and Japanese.
My parents’ home is in Ibaraki prefecture.