Based on the exuberant praise, and taken in context with Psalm 46 and 47, this Psalm also celebrates the victory the Lord gave Jerusalem in the face of destruction by the Assyrians (cf. 2 Kings 19, 2 Chronicles 32:1-23, and Isaiah 36).  This was a miraculous victory!

Psalm 48 is a song about Zion, the city of God, and its great King. In praising the Lord who loves Jerusalem, the psalmist sang of the city’s glory and security because the Lord delivered it from the enemy. On the basis of this, he offered praise to God.[1]

God is the Great King (Verses 1-8)
The Great King is worthy of praise (verse 1).
o   The Hebrew word for “great” (ga’-dol) means “heavy”.  The Lord is huge, magnificent, overwhelming, higher than the highest high, stronger, mightier…this is the Great King of Zion!
·         Th
e Great King is above all other gods
(verse 2).
o   “Zaphon” was said to be the dwelling place of Baal—the Canaanite god of fertility and had control over agriculture, animal husbandry, and human sexuality.
o   Verse two, then, is making an attack on the gods surrounding the Hebrews.  Instead of Zaphon being the upmost heights, Zion’s hill was.  Jerusalem, while being on a hill, was not the highest at all but figuratively, it was because that was the dwelling place of God Almighty.
o   Indeed, our Great King is above all other gods—greed, pride, control, Satan himself.  
·         The Great King is our protector (verses 3-8).
o   God is our fortress—our strong place, our shield, our defender.  His strength protects us!
o   Even though the entire world seems to come against us, God is the One Who defends us!
o   His power is absolute—even against what we perceive is insurmountable.

[1] John F. Walvoord, Roy
B. Zuck and Dallas Theological Seminary,
The Bible Knowledge Commentary : An Exposition of the Scriptures (Wheaton, IL: Victor Books, 1983-), Ps 47:7–9.