Bonus levels are small staircases that are randomly placed and do not show up on each lvl.
The 1000 gold are cumulative so you dont need to pick up 1000 in one bonus lvl.
when you enter a bonus lvl there will be 15 seconds to pick up as much as possible. If you hit a mine the bonus lvl will end.
I got my achivement after 3 or 4 bonus lvl's