I had 5/5 builders idle last week. I suppose that's what happens when you've been busy doing other things.. I'm pretty sure I will have idle builders for the rest of the game though, as I'm getting to the point where there are only a couple dozen more non-wall upgrades to go and they are all very expensive (6.5 million gold minimum).. wild stuff.
As for purple walls, keep it up everyone.. They are totally worth your while even if it is time consuming and boring... pink walls are simply too easy to bust through and you'll never be able to save enough for the more expensive upgrades just down the line without them.