What is a key ingredient in the Faulty Tower cocktail?

The key ingredient in the Faulty Tower cocktail is grapefruit juice. This ingredient is essential as it provides a vibrant and refreshing citrus flavor that balances the other components of the cocktail. Grapefruit juice adds a unique tartness that enhances the overall taste profile, contributing to the cocktail's character and appeal. In mixing cocktails, the choice of juice can significantly affect the drink's taste, and grapefruit juice is particularly valued for its ability to complement sweeter flavors, making it a popular choice in various cocktail recipes.