Question

What does tall and bold like a castle??​

Answer 1

a castle is usually a tall monument
it is related with the kings and it stands boldly and thus here it is used as a simili

Answer 2

So Basically Tall and bold like a castle, Looming Loom (verb) : to appear in a large, strange, or frightening form 1 in the dark. They rise to a towering height.