The brilliant design Derek has created with buildings is that the real costs to building a building is the opportunity cost it creates, like Ipisko mentions. If you build a building, you can't build troops. Also, there tend to be multiple things you can build at a given time. You're forced to make tough choices:
*Should I increase my mana per turn by building on a shard?
*Should I increase my income per turn by building a Merchant?
*Should I build a pioneer to found another city/outpost?
*Should I build troops to support my city/champions?
etc.
The other component in the mix is research. Research will unlock new buildings and troops, sometimes which will impact the other. For instance, in my most recent game I held off building troops until I built a War College for my Fortress. In the interim, my city was vulnerable, but that was a risk I took in exchange for a longer term investment.
The more you focus on building buildings, the less you focus on the short/near-term. If you do this too much, you get destroyed. If you don't invest enough, you'll also get destroyed. A very delicate balance there, especially with the mechanism of using the city on idle to generate growth, mana, money, or research. Depending on your goals, you'll choose different things at different times to exploit the resources and advantages you have.
