I would think that it is a safe place, a place to be able to fight from and not give up any ground to the enemy. Since I didn't know - I googled it! My concept was pretty close. It's what I would call a fort; definitions I read said it was a fort made of wooden stakes and usually located on a hill. But it also said that a castle was one of the most common types of strongholds. And get this - who lives in a castle? "the king or territorial lord."
Okay - now I'm rolling with it! God is our king! He is also the territorial Lord of my whole being. He won this territory of my soul a long time ago as I surrendered my territory to His Lordship! And now - I live in His castle with Him! I am crucified with Christ, right? (Gal. 2:20) I don't think I could come up with a safer place to be!
Psalm 48:3 says that God has made Himself known as a stronghold. I pray today that we all learn to know Him as our stronghold.