How do I make a Creeper proof city?

Depends how much work you are willing to put in :). It'sNotALie's suggestion of an obsidian city is viable but ugly, personally i keep light levels up using torches everywhere to prevent creepers spawning and then wall the city in so that they cannot enter from darker areas

Takes a lot of torches but it works, good luck :D

If you want, you can change the difficulty in the settings to peaceful. However, note that no more hostile mobs will spawn.

Creepers are afraid of ocelots

You can use this to your advantage and tie up a bunch of them around your city. While they are running from an ocelot, creepers will not explode. However, if your ocelot lags behind you and the creeper stops running they can still blow up.

Put glowstone everywhere because they can't spawn where there is light.

Go into the settings and near the bottom is a setting called “mob griefing” turn that off problem solved however NO ACHIEVEMENTS AFTER THAT! Other ways is proper lighting one torch every 12 blocks minimum different for other light sources including glowstone and redstone lamps less often and sea lanterns too.

Alternatively mobs won’t spawn on semi solid platforms so gates with carpets