What does the poem Eldorado mean by Edgar Allan Poe?

The poem Eldorado by Edgar Allan Poe tells a short story about a knight on an adventure to find the lost city of gold, Eldorado. The second verse explains:

"But he grew old -

This knight so bold -

And o'er his heart a shadow

Fell, as he found

No spot of ground

That looked like Eldorado."

This can infer that the knight's adventure went on for years, thus wearing the knight out. He met a "pilgrim shadow", whom he asked the true location of Eldorado. This is the Shadow's response:

" 'Over the Mountains

Of the Moon,

Down the Valley of the Shadow,

Ride, boldly ride,'

The shade replied, -

'If you seek for Eldorado!' "

The shadow could represent Death itself, and perhaps the shadow was referring to how Eldorado is impossible to find, and that the knight had wasted his life over a city that could not exist.