Here are some possibilities, depending on what you mean:
* Dense: "The forest was so dense, it was hard to see through the thick foliage."
* Defensive: "He became defensive when asked about his past mistakes."
* Intense: "She felt an intense feeling of longing for her home."
* Tenacious: "He was tenacious in his pursuit of his dream."
Could you give me more context about what you're trying to say, and I can help you choose the right word?