What are the poetic devices used in "The Eagle" by Alfred Lord Tennyson?

"The Eagle" begins with alliteration, a device Tennyson employs throughout the poem. The repeated "c" sounds also emphasize the suggestion of hypallage in "crooked," which applies to the eagle's talons and the crag itself. This connection helps show the connection of the eagle to its environment. The eagle is also "personified" by the use of the word "he" (as opposed to "it") and "hands."

The second line, along with using more alliteration, employs hyperbole. From the perspective of one looking up at the eagle, he appears close to the sun, whereas he is really only fractionally closer than the observer. This perspective, which centers the eagle and emphasizes his majesty, is continued in the phrase "Ringed with the azure world." This phrasing implies that the whole sky is merely providing a frame for the eagle.

In the second stanza, the slowness of the "wrinkled sea" crawling is contrasted with the sudden speed the eagle can muster. Again, the eagle is personified by the fact that his perch on the mountain has walls like a castle. However, the final image is of the eagle as a dynamic force of nature. Tennyson uses a simile that compares him to a thunderbolt. In Greek mythology, the eagle was the messenger and companion of Zeus. Zeus would communicate his displeasure to mankind by hurling thunderbolts, so this final dramatic image associates the eagle with divine power.

What are the Romantic features in "The Eagle" by Alfred, Lord Tennyson?

Romantic literature tends to focus on the wonders of nature, the importance of the individual and his or her experiences, and intense emotion as a means of establishing truth.  "The Eagle," by Alfred, Lord Tennyson, certainly presents all three of these qualities in some way.

First, nature is described with a great deal of visual imagery: "crag" instead of hill or even mountain is so much more descriptive; "Close to the sun" and "Ring'd with the azure world" are also grand visual images that help to inspire a sense of awe.  Even "wrinkled sea" is an unusual but effective way to describe the motion of the water as seen from a great distance.  The poem's visual imagery helps us to see nature as awe-inspiring.

Second, the power of this creature -- the eagle -- is evident as well.  Though he is not described as good or benevolent in any way, it is his singular and particular experience that makes up the poem's subject matter.  He is immensely powerful, more powerful than anything else.  And if we read the eagle as a symbol for a person who enjoys absolute power, as the eagle does, then the focus shifts to the experience of such a person.  If a person, the word "crooked" implies his corruption, and the "wrinkled sea" that "crawls" far below him can come to stand in for the masses of powerless who he controls or rules.  

Finally, "lonely lands" implies an intense awareness of one's utter solitude inspired by the eagle's power and position.  Because no being is as powerful as he, no one can enjoy the station that he does, the view he obtains from being so high above everyone else.  This symbolizes the way in which an absolute ruler will not share his power, and so he is forced to be alone.  To share his position and gain company would be to share his power and perhaps render him more liable to "fall."  We learn this through the beautiful visual images of an elite nature that only one in this role can enjoy, but, because he is alone, it is rendered somewhat less enjoyable.