In Civilization War, you can build various buildings within and around your Castle.

To build, upgrade or demolish any building you must use 1 Construction Hammer. The item is used automatically when you select the action, and does not need to be restored after usage, as you possess one at all times.

On the left side of the screen you can see the time remaining until the building action is complete.

There is also an additional Golden Hammer icon below the normal Hammer. You can use a Golden Hammer in case you wish to construct, upgrade or demolish more than one building simultaneously.

NOTE: In case you already possess freely-obtained Gold Hammers, you can use them directly from your bag.

Gold Hammers used from the main screen or building screen will be purchased for 250 Gold Coins.

To instantly complete any building action, you can also use Gold coins or as many Speed-up items as required in case you possess them. 

More details about Castle and level requirements to open buildings can be found in this article.