On Level 20 towers will no longer require the normal resource (Solid Bronze), but instead will demand Research Guides. This will increase the quality of the tower.

Overview of needed Research Guides
Tower Quality Research Guides Expert Research Manual
Common (Level 20) 30
Excellent-1 (Level 30) 100
Excellent-2 (Level 40) 200
Elite-1 (Level 50) 400
Elite-2 (Level 60) 800
Elite-3 (Level 70) 800 100

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