What does my alliance need to do in order to occupy a landmark?

Your alliance territory has to touch the plot occupied by the Relic (using hub or towers). Then you wait for the shield to drop (there is a timer of 2 hours when it is down) and attack the Relic. Careful, you HAVE to attack in a rally of strong APCs to punch through, otherwise the defending zombies regenerate between separate attacks (even 1 second apart). Don't bother scouting it, no info is given.