Create chains of 3, 4, or 5 mana runes. This will grant you resources that you can use towers. Once you have enough resources, build towers and place them in the map. These towers will help protect you from the enemy hordes. You should constantly be trying to earn mana and building more towers.
Most waves are ground-based, but a few have air units. Some towers can hit both, while others are more specialized. Hovering over a 'Tower Button' will bring up a tooltip with a short description of what it does.
Victory is achieved when Wave 30 is reached. Defeat occurs if 20 enemies slip past your defenses.