The Summon Circle

"If you need to help your towers really pack a punch, this Summoning Circle is the one to call." - BTD6
The Summon Circle is an end-game tower in BTD6, costing 5000$ to place, and has 1 upgrade path with only 3 more upgrades.

Ability - Sacrifice and Summon
This ability is given upon placing the tower. In order for it to activate upon pressing the ability button, you have to put monkeys on it in a specific combo in order for the sacrifice to work. The sacrifice combo book is in game and the ability to open it is also given upon placement of the tower. When you first open the book, it will be empty. This encourages players to experiment. When finding a new combo that was used for the first time, the book writes it down and when opened, the new combo can be found in the book.

Table of Combos.
Disclaimer : You can still perform Tier 0 Spells on a Tier 3 Summon Circle.