Castles turned senior in 2018. She competed at the British Championships as a guest and finished in fourth place amongst the other guests. In June Castles competed at the Nordic Championships where she helped Sweden finish second behind Norway. Individually she won gold in the all-around, on balance beam, and on floor exercise. She next competed at the Swedish National Championships where she placed first in the all-around, on balance beam, and on floor exercise. Castles was selected to represent Sweden at the European Championships. While there she qualified to the floor exercise final where she finished seventh. She next competed at the Paris Challenge Cup where she qualified to the balance beam final and finished in seventh place. Castles was selected to represent Sweden at the 2018 World Championships in Doha. During qualifications she finished 47th in the all-around and 39th on floor exercise; she did not qualify for any event finals.