I played Cut Through and didn't think it was so bad. Then I realized that I could only place one tower. I thought it was hard then, but when I noticed that you only get one life I started to get a little scared. xP
@Jeworski: The only way to actually get a Colonel rank is to beat a level perfectly on hard. What you're getting by beating Challenge mode is something better than a Colonel rank, which also works for the in-game achievement and the badge.