I was doing a search for old-school games to revisit and emulate for the sake of nostalgia when i came across a list of "The Top 10 Hardest Sega Genesis/Mega Drive Games" and at the number 4 position was Gaiares.
I have extremely fond memories of this game because even though it was on a 16-bit system, the simulated 3-D backgrounds, the gameplay, and especially the extremely well-done soundtrack were just so outstanding for the 1990.
The game is obviously influenced and similar to Gradius, Thunder Force, and R-Type but was unique in the sense that your weaponry was acquired by "stealing" it from any enemy that you encountered. Obviously some enemy weapons were better than others so it had a bit of a learning process involved.
For me, the major attraction of this game was the massive bosses who would likely kick the crap out of you the first couple of times that you encountered them but I think that is kind of the point of most bosses, isn't it?
Your ship could take multiple hits once powered up and you got 5 "lives" before needing to continue. The continues were also limited but I don't recall how many of them you got. I know it was few enough that victory was not guaranteed.
Now here is the part that gets me: Most of the reviews and writeups that I see have this game listed as definitely one of the most difficult games for Genesis if not one of the most difficult shooters ever made. I don't consider myself much of a hardcore gamer but perhaps I was back in 1990 because I recall defeating this game regularly. However, looking back at some of the footage maybe I was a hardcore gamer as a kid... i'm certainly not now.
If you want to experience the difficulty of this game for yourself you can try out the ROM, for free here