One iOS game I enjoyed and recently rediscovered is GeoDefense. I highly recommend it (at $3.99 I think) and it is definitely a middle-ground app. I estimate they made between 500K - 5M lifetime sales over 4 years with both games.
The geoDefense games (regular and swarm) were huge favorites of mine at the time. I wish they were still making games but haven't seen anything new from those developers since that game came out.
Man, those games were fantastic. Best tower defense games ever made by a long shot.