If you have resource spells (made from runes of light, the green ones) you can cast them on yourself or other alliance members. In map view, click the city and pick the flame symbol then pick the spell you want to cast, in the list on the left and click 'cast' at the bottom right.
You can cast damage spells the same way by picking an opponent or camp and casting spells on that. These are listed in the bottom left. Damage spells on your own city will use mana but not kill anything (unless you are under attack) and will also not fill your cloud.