“water shaft.” The Jebusite city was built on the south end of Mount Zion and its main water source was the Gihon Spring, a spring on the southeast end of the city, just outside the city wall. The Jebusites had a shaft leading down from their city to the spring, and Joab used that shaft to gain access to the city and conquer it.

“house.” The word “house” could refer to the house of the king, the palace, or the house of God, the Temple. The Jebusites were hated so they were not allowed into places where others could go.