I have been thinking about a world were everyone is shrunk to the size of a mouse, or at least the size big enough to ride a mouse, and I was wondering what are your ideas about what cities and architecture would be like in this world? Assume that the square cube law does not happen when the shrinking happens.
Architecture and cities would be exactly the same. Sadly, the profession of architect would be dead. Very little new urban construction would take place. Basically because neither would be required.
Once the human population was reduced to that mice, the whole business of human habitation would be transformed radically. Humans would have to accommodate themselves among a vast host of gigantic buildings and houses in which to live. There would be no need to build new cities or houses. Mouse-sized humans would be preoccupied with building appropriately sized accommodation inside those now gigantic houses and buildings.
This will require reorganization of society on an enormous scale to be able to live and work within human-sized structures. For example, a single house could now accommodate several families at once. People will need to move closer to facilities and work sites. Tiny people will need to trek from the outer suburbs. Power systems will fail because small people won't be able to run and maintain them. Effectively the technical level of the new shrunken humanity will fall. This explains the loss of technical professions such as architecture.
They will use existing structures for habitation and accommodation. For example, they won't be able to use stairs and elevators to go up and down in multi-story buildings. They will need to create tunnels in the form of shafts or ladders to go up and down.
If it is assumed that shrunken humanity remains mouse-sized, it will be centuries before they can develop the techniques necessary to build new cities. But during those initial centuries they will occupy niches within the former cities created by the former supersized human species.
In conclusion, do not expect a mouse-sized human species to be in the business of building new cities or having architects and and architecture. It will take centuries for those aspects of shrunken human civilization to emerge. When it finally does, it will be primarily based on using and recycling materials and structures made by bigger, former humans. Once new industries are created at the appropriate scales, they can begin creating cities and an architecture of their own. Since this will reflect their culture and civilization and because this will be considerably different from our era it will be different from ours. As such it can be only speculative. This will doubtless reflect a fascination with the former big human civilization, but executed on a smaller scale.