This one's easy enough: Mariana is at her farmhouse, and at her farmhouse is where she stays. This scene is set from the get go: moss overruns the walls and ceiling, rust is eroding the nails, and everything is dirty, dank, and dreary.
And that's just in the first stanza. In the second stanza, she peers outside. It isn't until the third stanza, in fact, that she ventures out. There she sees a barren landscape, save a poplar tree and a very misty landscape.
In stanza four, we see the moat that surrounds the farmhouse. It's dark and thick. It, too, isn't moving much. Nothing really moves, actually. In the sixth and seventh stanzas, Mariana returns to the house, and we see that it is inhabited by mice, noisy clocks, and the ghosts of her past.
Yikes. It's safe to say that the setting of the poem is a gloomy, dreary, unpleasant one—just like poor Mariana's life.