"The gates of hell shall not prevail against it" has generated vivid imagery, but the underlying expression deserves careful attention. Matthew uses Hades, the realm of the dead, rather than the word Gehenna.
Gates were also a natural image of a city's strength and enclosure. Together the terms create a picture of the power of death unable to overcome what Jesus is building.
Hades in Greek usage could denote the abode or realm of the dead and was used in the Greek Scriptures to represent Sheol. It should not automatically be equated with every later idea associated with the English word "hell."
Ancient city gates were substantial defensive structures and could symbolize the strength of the city they protected. The expression therefore evokes the apparent strength and confinement of death's realm.
The background changes the mental picture. The saying need not be imagined as the church hiding behind its walls while demonic gates attack it. Gates normally defend and confine.
Read against Jesus' coming death and resurrection, the promise that Hades will not prevail becomes especially powerful: death itself will not defeat the community Jesus builds.