For those who play for free, after a year of regular play my palace is at level 24 and I have accumulated 54k bronze.
Increasing VIP level, giving priority to hero advancement and skills which improve Uber killing plus spending gold on energy seem like the available ways slightly to increase the rate at which bronze accumulates.
My guess is that the time taken to accumulate enough bronze to continue any very extensive building programme beyond level 27 is measured in years rather than weeks or months.
So the challenge is to set targets and organise satisfying gameplay otherwise than through town development once level 27 is reached.