A nice feature when creating a level would be the option to start at a certain level, instead of always having to begin from the start. This way, when your testing a large leveled map, you could could fine tune the later levels without the time spend reaching them.

I like your idea pretty much. It should be similar to the "Flag" from the game "Line Rider". If you activate the "Flag", you'll start at this point, with all creeps, money and towers, that were on the screen.

For those who don't know Line Rider, here is the link:

http://www.spielkarussell.de/spiele/geschicktheit-spiele/line_rider_line_rider_2.html