Great game, one question – Kingdom Rush Frontiers HD Review

The game and gameplay is great, ironhide is one of the best game studios I've ever seen. But i do have one question, whats the difference between frontiers and frontiers HD? I found that a long time ago i bought HD on my old iPad and now i can download it for free, but what's the difference? I wanted to know wether or not i should buy the original frontiers or just keep hd?
Review by Don don12 on Kingdom Rush Frontiers HD.