The gates of a city were for a defense.

"And I say also unto thee, That thou art Peter, and
upon this rock I will build my church; and the gates of hell shall not prevail
against it." - Matthew 16:18 KJV

Why will the gates of hell not prevail against the church?

Because the gates of hell cannot prevail against the gospel
(Acts 2:38)

Souls are set free from hell by Jesus name!

"Then Peter said unto them, Repent, and be baptized
every one of you in the name of Jesus Christ for the remission of sins, and ye
shall receive the gift of the Holy Ghost." - Acts 2:38 KJV

 prevail against the gospel(Acts 2:38)

Souls are set free from hell by Jesus name!