# In that day

"At that time"

# this song will be sung in the land of Judah

"the people in the land of Judah will sing this song" 

# We have a strong city

This refers to the city of Jerusalem.

# God has made salvation its walls and ramparts

God's power to protect and save his people is spoken of as if his salvation were walls around a city. 

# ramparts

Ramparts are walls built around a city to keep enemy armies from getting into the city.