Need help placing towers

[bisfrim]

I'm trying to make a tower defense game, i got all my tiles setup, but i need help with selecting a tower and placing it anywhere in the tile.

This is an example of my tower class:

class Missile(pygame.sprite.Sprite):
def __init__(self):
pygame.sprite.Sprite.__init__(self)
self.cost = 100
self.selected = False
self.image = pygame.image.load(os.path.join('images', 'missile.png'))
self.rect = self.image.get_rect()
self.center = (520, 86)

def update(self):
self.rect.center = self.center