Fort Apocalypse. A classic 8-bit game in which you control a helicopter and have to fly around a cavern and rescue people while everything is trying to kill you. Like in Choplifter, to which this game could almost be a sequel, you either shoot in front of you, or you can turn and drop bombs to take out ground targets. It's actually pretty short, being only a couple of levels long (enter the underground, go down to another level and destroy a central reactor, then escape), but it's quite challenging.
As I've generally found with Synapse's games, I got on with the Atari version a lot more than the C64 because the Atari is a lot smoother to play. I just couldn't adjust to the choppy framerate of the C64. I do wish the game could have stolen Choplifter's controls. In Choplifter you switch weapons modes through deliberate action and stay in that mode until you want to change, but in Fort Apocalypse you switch based on how long you hold a direction down, which means that you come out of bombing mode as soon as you need to move, which you're going to have to do because stuff's always coming at you. It makes an already hectic game even more so.
Cathedral. Do you like Shovel Knight but wish it took after Metroid more? Here you go. It uses the same jacked up NES look and sound as you control an armored guy running around, collecting power ups to get to new areas. It's got a good challenge to it, hard enough to make you work and figure things out but not quite enough to make you yank your hair out.
I wish it had a better quick travel feature because I often found it advantageous to simply let my guy die to warp to the last save point, and I don't know if it's just my system but there's an annoying bug that would cause my controller to glitch out and go to the pause screen after I'd pushed the jump button occasionally. Also, for a game that uses this fake 8-bit style, it's weirdly system-intensive. I think my fans were going harder than they do when I play Prey or Bloodstained, which I've also been playing recently. But otherwise a very solid game.