Anno 2070 offers the perfect synergy of RTS and City Building. I recommend the complete edition because the Deep Sea expansion adds a layer of extra depth to the tech trees.
In Anno 2070, the size of your city is limited only by the size of your island--and there are some really BIG islands. If for some reason you aren't satisfied with your island, build some nukes, launch them, watch the pretty mushroom clouds and then move into your newly acquired glass parking lot (island). Don't feel like remediating nuclear radiation from your newly conquered paradise? Try sabotaging your enemies' main food source--destroy an oil tanker off the coast of an enemy island (preferably next to their fish hatcheries) and while he/she scrambles to clean up the oil, harass the cleanup crew with fast attack boats or helicopters and then send in your attack fleet. Even better, with the Deep Sea expansion, you can create tsunamis to wipe out your enemies' coastal holdings by destroying strategically placed geothermal power plants. After all, your citizens are worth more than your enemies' citizens, especially if your enemy happens to be the Eden faction... those hippie tree-hugging ♥♥♥♥s! Pick the Tycoons and use the power of coal and nuclear energy to crush the little Eco ♥♥♥♥♥! There's no Captain Planet to help them against your rain of nuclear fire! Crush them and their tree covered islands and pave over everything with concrete!
Honestly, this game is a blast! The only reason Anno 2070 doesn't have 'Overwhelmingly Positive' reviews is because people hate on the Uplay DRM. I agree that its annoying but, its no reason to avoid one of the best city builders/rts hybrids of all-time.
EDIT: I noticed a number of people complaining that Uplay doesn't work for Anno 2070. Since I hadn't played in a few weeks I decided to boot it up and see if there was a problem. Uplay works just fine. The Steam version of Uplay is outdated. You need to update Uplay and everything works just fine.
The fix for this is so simple its mind boggling how many people fail to see it when its mentioned in numerous threads within the community hub. I know the Steam fan-boy knee-jerk reaction is to hate/blame any third party DRM so I guess I shouldn't be so surprised.