This is reiterating what Dallas1195 said, but communicating the form understandably. If you click the world button on the bottom, and without exiting out of it click on another section on the bottom bar, such as progress bar, you will still see the houses from the World Section overlapping the screen. You can still exit out of it, however, using the X in the top right hand corner.
lol, all u need to do is build a long pathway with machine guns, with a few goop towers and electro towers, then get missile launchers spaced enough that there is at least 1 attacking every spot on the field... pretty decent, but if u know tds, u dont need to try that much
i feel like this is a demo, and that there should be a better game where u can upgrade everything more, theres 100 levels, and u can have a heck of a lot more fun