Using a sharp knife and cutting board, cut the watermelon into 1-inch thick horizontal slices, ensuring each slice is even and stable.
Select the most uniform slices and remove the rind carefully with a knife, leaving the pink flesh intact. Slice off any uneven edges to create flat, clean surfaces on the top and bottom of each slice.
Arrange a large round plate or cake board. Starting from the bottom, place the first watermelon slice flat on the surface. Use a spatula or your hands to gently push down and stabilize the slice.
Layer the subsequent 1-inch thick watermelon slices on top of the first, pressing lightly to ensure they stick together and form a stacked, cake-like structure. Continue until you have used all slices, creating a tall, singular pile that resembles a cake.
Once assembled, chill the watermelon cake in the refrigerator for at least 10 minutes to firm up and help the slices adhere better. The finished structure should be firm, with a uniform pink color and smooth sides.
Remove from the refrigerator, slice carefully with a sharp knife if serving in portions, and optionally garnish with fresh mint leaves or fruit for presentation. Serve immediately to enjoy the juicy, sponge-like texture of the watermelon slices.