Every inch of air and every hill surrounding the redoubt is constantly under the watchful eye of Lira's drakes. They must have found a way into the shrine from beneath.
Take this Flashgrowth mote. It should prove more than sufficient to close any hole, tunnel, or portal they might have used to get inside our defenses.
Find the breach in our defenses. Close it.
Completion
Well done. Thank you, <race>.
I will not be so foolish as to let such an oversight happen again.