The wrinkled sea beneath him crawls; (line 4)
The words "wrinkled" and "crawls" seem to personify the ocean, or at least make it seem like a living thing. The speaker tries to see the world from the eagle's perspective, but he still comes up with human comparisons.
He watches from his mountain walls, (line 5)
The eagle probably has his nest on the side of the mountain. He surveys his surroundings like the lions on Pride Rock in The Lion King. Come to think of it, Pride Rock is also a "crag."
And like a thunderbolt he falls. (line 6)
The eagle falls like a burst of energy in the atmosphere. This comparison is appropriate because Tennyson has been setting up a contrast between the sky and the earth in the poem. The final line brings sky and earth together; or, more accurately, the power of the sky comes down to earth.