The tower discount upgrade seems buggy, as there were a few levels were I wasn't allowed to put down a tower until it reached the non-discounted price and it would then also cost the non-discounted amount.
Like the game. It'd be nice to have something to work towards, though. Maybe three difficulty levels: Easy go at least 500 miles, Medium go at least 1000 miles, Hard go at least 1500 miles. You could adjust those goals based on what the scores are showing you.
Also, pipes need to be able to twist both ways (clockwise CLICK, counter-clockwise SHIFT+CLICK). Also agree that a timer shared between levels is not fun.
Bug fixed. Thanks for reporting!