if you buy a tower and hold down on it, bring to a spot where it could be placed and right click, you can put a tower anywhere on the map if you left click fast enough. yes you can towers on the track and inside of each other. it
makes the game a lot easier
if you are having trouble make a game on easy with mods on to get all the meddles but the days survived ones then try again with the perks and it should be easier