I am working on an AI bot for the game Defcon. The game has cities, with varying populations, and defensive structures with limited range. I'm trying to work out a good algorithm for placing defence towers.

• Cities with higher populations are more important to defend
• Losing a defence tower is a blow, so towers should be placed reasonably close together
• Towers and cities can only be placed on land

So, with these three rules, we see that the best kind of placement is towers being placed in a ring around the largest population areas (although I don't want an algorithm just to blindly place a ring around the highest area of population, sometime there might be 2 sets of cities far apart, in which case the algorithm should make 2 circles, each one half my total towers).

I'm wondering what kind of algorithms might be used for determining placement of towers?