Minor Update 667(iOS)/668(Android). This update will require a game restart.
Minor Update 667 will restore the previous relationship between defensive tower levels and player levels that existed prior to tier-based discounting, based on Storage Hut requirements. The intention of Tier-Based Discounting is to make certain tower levels achievable faster and requiring less resources, but it was not designed to make any particular tower level accessible to players at a lower player level than before.
To remedy this, we are copying over the player level restrictions from the storage hut directly to the towers themselves. The end result will be that all tower levels are unlocked at the same player levels as prior to the release of Tier-Based discounting.