You just need to make sure you have enough units defending your cities to prevent them from being taken over, especially if you want to pursue a culture win. Always helps to have them city defence promoted, and have no less than 4 in each city, although more would be preferable. If you want to have an active defence then have some mounted units, or tanks if you have them stationed in the cities to conduct counter-attacks on units attacking your city. Having ample military units is usually the best counter to aggressive civs, i guess it deters them. Of course, military units are going to cost you, but the cost doesnt have a major effect on the game.