Tile improvements are structures that you can build around cities to enhance the value of non-city tiles. These improvements range from <l:8,0>farms<e> (which increase <l:6,90>food<e>), <l:8,3>mines<e> (which increase production), and <l:8,19>roads<e> (which reduce unit movement costs) to military installations. Tile improvements can only be built by generating <l:6,61>public works<e>.