Which way is the enemy's gate? Why is this important?

The enemy's gate is "down."  This of course refers to the
Battle Room at Battle School and a tactical approach to re-orienting all soldiers so
they have a common goal and a common sense of direction in a cubical room which contains
no gravity.

While at Battle School, Ender quickly realizes
that most commanders rely on the direction of gravity as it was in the corridor when
they enter the Battle Room.  This means that most of the time the enemy gate is directly
"in front" of their gate.  What Ender proposes when he instructs his army to imagine the
enemy as "down" is genius.  First, they immediately "drop in" on the enemy, presenting
only their feet as moving targets, which are very small and hard to hit.  Second, and
most important for later, this "dropping in" on the enemy simulates the attack of a
planet from space.  No matter what part of the planet his army would "land on" they
would all have to move "downward" to reach it.  This proves to be the exact solution to
the final battle on The Simulator when Ender is at Command
School.