Antonym of nature, that implies the cities and infrastructure constructed by man
Solution 1:
Civilization would seem to fit your example.
As in, "I took a brief retreat from civilization to enjoy the Earth gifted to us by Nature."
Solution 2:
the built environment
built environment
noun
Man-made structures, features, and facilities viewed collectively as an environment in which people live and work.
‘architects who are helping to shape the built environment in this newly prominent region’
Oxford Living Dictionariesthe built environment
the buildings and all other things constructed by human beings
Collins Dictionarythe built environment
noun
all the structures people have built when considered as separate from the natural environment
Macmillan DictionaryIn social science, the term built environment, or built world, refers to the human-made surroundings that provide the setting for human activity, ranging in scale from buildings to parks.
Wikipedia article
You can also go with artifact. Artifacts are things made by humans.
n.
1. An object produced or shaped by human craft, especially a tool, weapon, or ornament of archaeological or historical interest. 2. Something viewed as a product of human conception or agency rather than an inherent element
American Heritage Dictionary
Solution 3:
Maybe try infrastructure.
noun 1.3 the system of public works of a country, state, or region; also : the resources (such as personnel, buildings, or equipment) required for an activity
"Infra- means 'below;' so the infrastructure is the 'underlying structure' of a country and its economy, the fixed installations that it needs in order to function. These include roads, bridges, dams, the water and sewer systems, railways and subways, airports, and harbors. These are generally government-built and publicly owned."