Shrouded forms that start and sigh
As they pass the wanderer by —
White —robed forms of friends long given,
In agony, to the Earth — and Heaven.
For the heart whose woes are legion
'Tis a peaceful, soothing region —
For the spirit that walks in shadow
'Tis — oh, 'tis an Eldorado!
But the traveller, travelling through it,
May not — dare not openly view it!
Never its mysteries are exposed
To the weak human eye unclosed;
So wills its King, who hath forbid
The uplifting of the fringed lid;
And thus the sad Soul that here passes
Beholds it but through darkened glasses.
We do not eventually figure out who Dream-land’s king is. Perhaps he is the  Eidolon named NIGHT  from the first stanza, or maybe even God himself. Either way, the King forbids for travelers to see how the true city looks like.
Dream-Land