Nukes are cities where you only build one kind of troops. For instance, a slinger nuke is a city where you only build slingers; a bireme nuke is a city where you only build biremes etc...
The most effective nukes in my opinion are slinger nukes, swords nukes, harpy nukes, manticore nukes, ls nukes and birme nukes. Only ones I've ever built.
Make sure if this is your first city to not build up an offensive nuke other than to take out farming villages.
Once you hit 5k city points, you then become a target to bigger players who just want your city and if its full of nukes, they will wipe them and your city out with little effort
Its safer to build nukes when you have more cities to support them.