stoe will take forever and will not give you the look you probably have in your mind.
I would do this
then I would do it again, styrofoam is cheap.
Then I would do it again. practice makes perfect and you will learn A LOT each time you do it. plus it is just kind of fun.
Then the last time do it with aquarium grade silicone and drylock paint that has been tinted with concrete tint. Take your time and before you are all done put your aquarium light over your castle because it will look 10x lighter...so add some more really dark paint and check again.
20L My Other Tank: OEBT Shrimp
40B Inlaws Tank: 2 goldfish
75 Main Tank w/ sump: 5 Glossolepis pseudoincisus, 5 Melanotaenia boesemani, 10 Melanotaenia praecox, 5 Melanotaenia lacustris, some cories, a bn pleco and rainbow fish fry