well it has a lot a words inside it
One word. 'Sandstorm.'
Yes the word sandcastles is a noun. It is the plural of sandcastle.
Sandcastles
sandcastle
Yes
"To build a sandcastle" in Spanish is "edificar un castillo de arena." It literally means "to build a castle of sand." There are a host of English words that are composed of two or more smaller word, e.g., sandcastle, stardust, pigpen, classroom, etc., which are grammatically not done in Spanish. Unlike English, Spanish doesn't [normally] put two nouns together and pretend that one modifies the other; at least to a far, far lesser degree than English does.
By themselves, no, they are not, but if you put them together to form the word sandcastle, that IS a compound word.
Yes, sandcastle is a common noun.
The sandcastle is swept away.
Operation Sandcastle happened in 1955.
The sandcastle one
The cast of Sandcastle - 2013 includes: Katy Fullinwider