The Last Bastion Of Mana is a "Tower-Defense" style game where your goal is to survive as long as possible, keeping your city alive by building up your defense with walls and various towers. There is no "cost" for these towers and walls, as you can place as many as you want, but be wary - the more of each defense you have placed, the weaker they get!
You can repair the strength of your defenses with the points you earn from destroying enemies using the arrows next to the tower's name (5 points per upgrade).