Thank you both for your replies!

I was actually thinking ganache v poured fondant. How about that. I'm
smarter than I thought!
I would *love* to use a lemon glaze, but alas, the boy wants a
CHOCOLATE cake, naturally.
The sandcastle is going to sit atop a regular rectangular base. The
sandcastle part is for the kids,
covered with candy.. and visually impressive. The bottom part is for
the grownups. Same cake
in both parts.

So, I might just go with the fondant for the top so I can make it gray
like a castle... I'll use the pan as a reference and pipe on the
details that get obscured.

Then for the bottom I'll use something more delicious for the grownups.


Thanks again!
S.