"The Ememy's Gate is Down" is a catch phrase from Orson Scott Card's awesome Hugo and Nebula award winning science fiction novel Ender's Game.

The book is largely about kids who go to Battle School. There is an interstellar war on, you see, and all the earth nations collaborate to train kids to fight in it starting in grade school. The kids live at the Battle School, which is on a space station, and are divided into little tribal armies which become tight teams. The main activity and training exercise is a sort of Zero G Lazer Tag game, which becomes increasingly complex as the tale goes on.

A lot of the story is about learning to think outside of the box and see things from a different perspective. The Battle Room is a large rectangular space, like a gym but in zero gravity, with an entrance for each team in the middle of the opposing smaller square walls. The goal is to reach the other team's door without getting tagged out. At first they all tend to enter facing the opposing team, aligning themselves to a preconceived local vertical. One day Ender (the hero) comes up with a new stategy: Imagine that the enemy's gate is down, and fling yourself towards it feet first, pointing shoes rather than chests at the enemy, and thus presenting a smaller target. They handily win the round and change the game forever.

This is the surface meaning of "The Enemy's Gate is Down", though as usual with such gems, the true sense of it runs much deeper. It's one of our oft repeated battle cries. Unlike most of our oft repeated battle cries, it's mostly said internally. What it means is that "The Enemy" is more vulnerable than it appears. By which, seeing as how another of our oft repeated battle cries is "No Enemies!", I mean the challenge we're facing is more doable than it feels.

Succinctly, The Enemy's Gate is Down is a mix of Think Outside the Box and Yes We Can.
It is an oft cited cultural reference. A favorite example