I guess "fortress" is a rather guarded, defensive way to imagine it.
I am wearing bright green sneakers as I write this.
"poetry is the joining of two strands". With that, Mandelstam had something specific in mind, but there are other ways of applying it.
Like thinking of poetry as displaying the harmonious union of technique and genius (inspiration). "she makes it look easy" : because the technique is driven by an overwhelming impulse or motivation, an ordering impulse. magnetizing, harmonizing, expanding, concentrating.
separate, separate, separate from the everyday.