Adjectives are parts of speech that modify nouns and pronouns. A helpful way to check successfully for adjectives is to ask these questions: What kind? Which one? How many? How much? Whose?
When looking for an adjective in a sentence, one of these questions will be answered by the adjective. In this specific sentence, the words “thick,” ”iron,” and “visible” function as adjectives. All three of these adjectives are attributive adjectives, because they add specificity and description to the words they modify and to the sentence as a whole. When examining this sentence, the articles ("a" and "the"), which are sometimes counted as another type of adjective, could also be identified, although the attributive adjectives are certainly more interesting.
“Thick” modifies “tangle” and answers the question “Which one?”
Likewise, “iron” and “visible” both modify “gate” and answer the question “What kind?”