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.
- The AotVM acts like a Tier-6 tower because it requires 2 other Super Monkey Tier-5 upgrades, it's expensive and it's very powerful.
- Some of the Tier-5 upgrades have very basic names, often containing words like "Super" and "Master."