Elders' Square is located inside Stratholme Undead Side, just past the instance portal of the servant entrance. It leads to the Gauntlet north through a gate, and going through this gate will activate the 45-minute timer to save Ysida Harmon.
Another gate leads west to Festival Lane on the Scarlet Side, the only thoroughfare between the two halves of Stratholme. This passage is where Atiesh is summoned. In the middle of Elders' Square lies the Alonsus Chapel, its hallowed ground keeping the swarms of Scourge at bay.