Pinnacle hit the nail on the head. I've specialized my iron/lumber and food cities. I get ~1 mil per hour from the iron/lumber cities. Its not harder to obtain the valleys needed at all. Ive my 3 cities in one central location, with a 15mi stretch of npcs to hit, and plenty of valleys to play with. all are lvl 9/10.

He's also right on the cottages. Initially you get more from having tons of barracks, but as you farm lvl 10's and yer atk hero skyrockets, you'll begin to get hung up on your population. You can drop prod % to 0 to que up of course, but theres a point where using that, even with 9-10 cottages, won't work. Unless you have so little of a life(or are so dedicated to the game) that you race to find a computer to que up more troops every 3 hours or so and set your alarm clock accordingly. Basically I keep an eye on the ETA for a full que. If I que up all available population, and it takes less than 7 hours to finish them all; drop a barrack for a cottage until the que can last the work day or continue building overnight till you wake up and que more.