To add depth to a curry, focus on the foundational elements. Use a variety of spices and ensure they are fresh. Toasting the spices before adding them can release their full aroma and flavor. Incorporating ingredients like caramelized onions, tomato paste, or a splash of coconut milk can also add layers of flavor. Lastly, a bit of acidity, like a squeeze of lime or a dash of tamarind, can brighten the flavors and add complexity.