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.
