The word towering is playing the role of and adjective in the sentence. It is placed before a noun: cloud, so that it is describing and modifying it.
Adjectives are descriptive words that modify nouns. They provide additional detail. They can also describe the quantity of nouns: many, few, millions, eleven. Adjectives ending in ing tend to describe the thing that causes the emotion. Adjectives do not modify verbs or adverbs or other adjectives.