Analyzing the Word 'Sentinels' in Context

Read the following excerpt from a narrative:

"In the heart of the forest, the ancient trees stood like sentinels, their gnarled branches intertwining in a whisper of secrets long forgotten. A soft mist curled around their bases, veiling the underbrush in an ethereal glow. Within this enchanted realm, the air was thick with the scent of earth and moss, each breath a reminder of the untamed wilderness that thrived in silence. However, the tranquility belied a sense of foreboding, as shadows flitted between the trunks, hinting at untold mysteries lurking just out of sight."

In this passage, the word sentinels is used to describe the trees. What does this word choice primarily suggest about the trees?