From Desert Operations Wiki
The way you build will all depend on the Capital in the The Start. In the beginning you can only build a few buildings. The following Building that can be built can be in any of the open spaces. Once the construction of the building has been finished you can also rise the levels of certain buildings. If you want to construct something you should take a look at the Building-Techtree on the right side of the screen.
Some buildings you can construct many times with each construction the costs go up 20%. The price will be for one single building and when you enter different amounts it will all be added together. The time of duration will depend on the amount entered. Some building also have extensions that when built will bring you bonuses. For example a higher attack value.
The Durations and Costs are always bases on the first Building.