He watches from his mountain walls (line 5)
The eagle continues to "stand watch" over the landscape. The eagle's vision is a kind of skill – it can see things from much farther away than can humans. Still, the poem keeps the eagle's coolest skills in reserve, in order to build suspense.
And like a thunderbolt he falls (line 6)
Like a dive-bombing fighter plane, the eagle careens off the crag, and he relies on gravity to build incredible speed. His flight is pure energy. The verb "falls" pairs with "stands" at the end of the first stanza. "Standing" and "falling" are opposite actions.