This series peaked a while back – Sentinel 4: Dark Star Review

Sentinel 3, despite some balance problems in the upper levels of the game, was the most enjoyable tower defense title I've ever played. This is basically the same game but with IAP and a few so-so towers added. Graphics, chrome, sfx, and all the rest are barely changed from the last release. Bottom line: this is a waste of your time and patience if you've played Sentinel 3, and if you haven't then you should try that one first.
Review by jzipps on Sentinel 4: Dark Star.