Verses with the word castle in the Old Testament (3 verses):

1 Chronicles 11:5
And the inhabitants of Jebus said to David, Thou shalt not come hither. Nevertheless David took the castle of Zion, which is the city of David.

1 Chronicles 11:7
And David dwelt in the castle; therefore they called it the city of David.

Proverbs 18:19
A brother offended is harder to be won than a strong city: and their contentions are like the bars of a castle.