Perfect World International players will want to check into the game during scheduled live events throughout this week. The City Siege Event is slated to begin at 7:00 p.m. each evening when the Sentinels appear in each of the alliance races' cities. The event features six phases with different mobs of monsters invading the cities that players are charged with killing.
As you kill the monsters noted in your quest log, the log will tally how many of each kind you've taken down. The more you slay, the bigger your EXP/Spirit bonus will be when you actually turn in the quest.
With each city-specific phase, the monsters will grow stronger, and the rewards for killing them will increase as well, as the more powerful ones drop more valuable items.
At Phase 4, the battle will move to the game's magnificent neutral cities - Archosaur, Dreamweaver Port, and Thousand Streams. At these locations you will then have to fight through waves of stronger and stronger foes, again culminating in boss monster fights.
Read more about the event on the Perfect World International blog.