The Crystal City had methods to notice Noah and the others, but the Eternal Snakes claimed the full attention of their sensors. Moreover, those cultivators wouldn't mind the small pack in the air when they had middle tier magical beasts laying waste to their defenses.
A purple dome quickly covered the palace while other defenses shot out of the structure and terrain. More rank 8 golems and light appeared in the forest and clashed with the Snakes.
The Crystal City had a vast network. It gathered information both from organizations and magical beasts. Having to face a sudden assault was almost impossible.
However, the Rat had led Noah's group through secret paths. The Eternal Snakes could attack before the Crystal City could deploy its defenses in a proper formation.
The Snakes only had to defeat the few defenses that managed to appear on their paths before charging ahead. Their advance would have been far slower if the Crystal City had managed to deploy everything together.