Alma 49:20

Thus they were prepared; yea, a body of their most strong men, with their swords and their slings, to smite down all who should attempt to come into their place of security, by the place of entrance; and thus were they prepared to defend themselves against the Lamanites.

They were prepared. A group of their strongest men stood with their swords and their slings. They would strike down anyone who tried to come into their safe place through the entrance. This is how they were ready to defend themselves against the Lamanites.

They had their strongest men posted at the entrance—armed with swords and slings—ready to cut down anyone who tried to force their way in. That’s how they defended themselves against the Lamanites.