When it was released, I remember hearing that the few lyrics were in a Native language. I can't find which language right now.
Check out the video:
She combines sleek modern dresses with a breastplate and feathers. I presume she's supposed to be an Indian, and she may be one (she looks familiar). There's also a shot of tomahawks arching through the air.
I don't know if the video is sending a message. But with all the elements, we can guess at one. Seems to me it's saying that Western civilization may decline, but Native people and their values endure. While we're falling down, they're still dancing. Unlike us, they feel the earth's pulse, its music.
In other words, they serve as the "sentinel" of the title, watching over us like parents, hoping we'll grow up before it's too late. We took their land, so now it's our responsibility. We need to keep it safe, preserve it for future generations, as they would've done.
Or maybe I'm just imagining things. Who knows?
For more on the subject, see Oldfield's Native-Themed Songs and Oldfield's Song of Hiawatha.