Castella are a simple Japanese sponge cake made with sugar, flour, eggs, and starch syrup. It is based on a Portuguese dessert called Pao de Castela that was introduced to Japan in the 16th century. 
At the time of Castella's introduction, sugar was a rarity in Japan that could only be obtained through a trading port at Nagasaki. With time the Japanese developed their own sugar crops but it remained an incredibly expensive ingredient. As a result, Castella was an exclusive snack of Japan's aristocracy throughout the Edo-era.