Walls and towers are defensive structures, which are not able to train units, and are used only for the purposes of defense. They are able to research some upgrades, although these are generally only useful to the building performing the research. Another type of building available to players, is a Wonder: a grand building that represents an architectural achievement of the civilization. In certain game modes, once a player builds a wonder, a ten-minute countdown begins. If the wonder is still standing after the countdown ends, the player who built the wonder wins.