Tier-5 Upgrades are special upgrades in BTD6. Unlike most upgrade tiers, only one Tier 5 upgrade per path per tower can only be bought once per game (With the exception of the Crossbow Master if the Monkey Knowledge "Master Double Cross" is purchased). Tier 5 upgrades are generally similar to their Tier 4 upgrade, but significantly more powerful and often having a special ability unique to them. The icons and artworks for each Tier 5 upgrade are hidden behind a golden lock until they are unlocked through gaining enough Experience Points for any tower and purchasing them.
- When a Tier 5 upgrade is locked, in most occasions being due to maxing out in numbers of a specific Tier 5 tower, the upgrade description for the locked Tier 5 will be blank instead of giving a valid description of why the upgrade is locked.
- This has been patched as of the 4.0 update.
- Occasionally, the Monkey Knowledge "Master Double Cross" can cause a bug where the second Crossbow Master is not able to be purchased.
- Some of the Tier 5 upgrades have very basic names, often containing words like "Super" and "Master."