The word 'miraculous' implies Dharker's admiration for the people who live in the slums and create homes/lives there, emphasising their society is special and unique
Could suggest the hope and happiness the people create in the slums despite their circumstances, but 'thin' could represent the fragility and vulnerability of their society
The metaphor of the 'walls' could imply that their optimism protects them from the harsh reality of their lives, and Dharker ends the poem positively and with hope