To enter the build menu, tap the hammer icon in the bottom left-hand corner of the screen. In the Build Menu, you have three options: Economy, Defense and Support. These are fairly self-explanatory.
buildings contribute towards the running of your empire - allowing you to build up Gold and Wood, which are essential in the construction of other buildings and your armies.
is where you build all of the structures that protect your base - things like guard towers and mines.
contains the rest of the buildings and units - Landing Craft, Armory (for upgrading your units), and so on. Bear in mind that unlike similar games, there is no Barracks - instead, units are trained through the Landing Crafts themselves.
The Build menu is where you can construct any of the game's buildings