Yet Sara's mother already spread her masters over a huge area, making them isolated from each other. In addition to using them to activate the formation, William also depended on Sara's mother to scatter them over the area, separating them, and making it hard and pretty impossible to destroy the formation.
"Get ready," seeing what his enemies were doing made William evilly laugh, "our friend from far is about to get her warm welcoming ceremony. Do it!"
The formation was some sort of both sealing and weakening formations. Its main foundation was laid already out there, but the main controllers were selected masters from the guild.
William didn't interfere in picking them, yet he advised his friends to pick the gold grade masters, leaving the dark gold ones to fight the enemies. As for the weaker masters, their role was to support any needs both groups of masters needed.