On any turn you can play a (one) tower piece (instead of a meeple) on any
existing foundation (including one just placed) or on any
So, it is not necessary to play a foundation tile in the same turn. You can play a tower piece on the same turn as you place a normal (non-foundation) tile.
The tower extension makes the game quite flexible, and vicious
Just to clarify.. if there is a meeple on top of the tower.. you cannot build on the tower until the meeple is removed. The meeple can be removed by the Dragon from the Princess and the Dragon expansion.. or by another tower in the Tower expansion.