After reaching level 70 of any towers (Elite-III), for higher tier you need research materials. Unlike low tier advancements, from this point you also need Excellent Cornealians and Expert Research Manuals.
How to get one?
- reward from plundering the final level of Coastal City state (max. 40% chance)
- reward from plundering the lesser-boss level of Magic Cave (max. 40% chance)
- after reaching Village Level of 71, you can forge 5 Research Guides for 1 Expert Research Manual