Lift Up Your Heads, Ye Mighty Gates

We sing it every Advent. It's from Psalm 24:
Lift up your heads, o you gates,
and be lifted up, you everlasting doors,
and the King of Glory shall come in.

What gates?
Gates of heaven?
Gates of Jerusalem?
The doors of our hearts?
The pastors?
And where are these "heads"? On us or on the gates?
Even though I hear this often in hymns and sermons, I still don't know what it's about.

And then in Didache recently, we were discussing the Second Coming. In Mark 13, as He speaks of both the crucifixion and His coming at the Last Day, Jesus says, "When you see all these things happening, know that it is near -- at the doors!"

Doors, eh?

And then He tells them to watch: "Take heed, watch and pray; for you do not know when the time is. It is like a man going to a far country, who left his house and gave authority to his servants, and to each his work, and commanded the doorkeeper to watch." You have to lift up your head to watch for something.