Dark Castle by Silicon Beach Software, released originally in 1986 on the Apple Macintosh.
Now, here's one that really takes me back. This game was one of the few I actually played on a Mac Classic back in the day, my friend's dad had a machine and we'd get some games in now and again - this was far and away the coolest one he had (other than Tetris, and some Shuffle Puck Cafe game, I believe)
It was remarkable to play this, at the time - though it was black and white (most PC games were still like 4 or 16 colors anyway, so not much to look at!), the graphics were remarkably detailed. This game had a very unique look all it's own. Very "Mac" (for what that meant at the time, anyway).
Also, the game was chick full of great digitized sound effects, something else you never heard back then. It really stood out with the audiovisuals (even with lack of color). This was an odd, yet remarkable game.
So what of the game itself? Oh boy, it's been so many years - I forget he plot, maybe you were trying to escape from this castle, or find some treasure or something? Whatever the case, you control a little fellow who must avoid all sorts of traps and enemies - bats, creatures, vermin, other people - you could chuck rocks to stun them, and of course many of the usual platforming trappings were in effect here (running up and down staircases, jumping on swinging wires, etc). I never got to far through this - in fact, hardly played it, I just got a kick out of watching my buddy play it anyway - so I never ended up seeing too far through the game. It made a lasting impression of me however as "that cool Mac Game!"
Dark Castle was ported several years later for a console actually, it showed up in the early 1990s on the Sega Genesis courtesy of Electronic Arts. The port was not so well-received - it certainly was not what you'd consider designed with the console gamer in mind, for starters - and they didn't really bring the controls over as well. Also the whole look of the game got reduced to a drab, blah rendition. It actually looked quite inferior to the original version, even with the new inclusion of color. I don't recall playing it (I might have messed with it for a weekend, I don't really have a memory of it). Be that as it may, the Mac original is still a great game!