What mental image do these lines from the poem evoke? the domes over fall to pieces. the stones under fall to pieces. rain and ice wreck the works. castles debris concrete sand

Well, whatever image comes to mind depends on who is reading the poem of course. But what I see is something like this:

Old ruins, vast, wide, maybe nestled down in a little green valley. In contrast to the flourishing wildlife around it, the ruins are fallen, cracked, worn and old. Covered by vines, moss hiding elaborate designs, and the rain and ice having cracked and eaten away at the once proud castle ruins. 

This is just my personal image that comes to mind... it will definitely vary from person to person. :) You never know, others could be seeing pyramid ruins in the deserts or an old statue or palace sunk to the bottom of an ocean... Let your imagination do the work!

Conclusion is the answer