Many of the earliest games I had, were on console(s) and had to be played against a human opponent, to be truly enjoyable (so, it wasn't really the game that got beaten, but the human opponent).

Therefore I guess, the first instance of a game that I have beaten, must have been on the C-64.

Though which one was "the oldest"..?
I truly can't say with any determination, anymore.

Probably "Kaiser" (1984)?
I know for sure that I beat that one before "Uridium" (1986).

Now, was there any other "known" game before "Kaiser", that I have beaten, too?
Pffft...beats me (pun intended).

Thirst thing that presented its self was/is Homeworld. Before that, Warcraft II i think and or Command and Conquer.

i'm pretty sure I managed to finish scenario's in the ancient art of War done by Broderbund (1984). But not all of them and it is certainly not a game i own as a physical copy these days. I'm also pretty sure the version i played when i was at my uncle's was also copied... Like the army chipmunk he was.... so beaten, Attila, yes, Napoleon, probably... i forgot the rushing type general's name, but he was loads out of my league

I've beaten Akalabeth.

I've also beaten Ultima 1 and Wizardry 1. (Although, for Ultima 1 I've only beaten the remake (one if the first remakes ever), and for Wizardry 1 I believe my only clear of the original Apple 2 version used a glitch to basically skip the entire game.)

I haven't played the original Doom because that type of game just never interested me.

(I ended up not liking what I played of Metroid Prime, which is the closest to that genre I've played. Well, and playing Dialog 3D, but that's more of a satirical game than something that's meant to be played seriously, and I didn't get far.)

You should check out Ragnarok from the same era, if you haven't. Like CotW, it was a shareware NetHack/Rogue/[whatever]-inspired game with tile-based graphics and mouse support. It doesn't have as much focus on story as COtW, though, and is less structured in general (and harder -- and I found the final version most commonly found as a download these days to be harder still than the older shareware release I originally played, thanks to the addition of one annoying enemy in the early areas). While it does have sound, it's PC speaker only, and used very sparingly...which is probably a blessing. (I guess there was a slightly different non-shareware commercial version released in Europe, under the name Valhalla, which had music and proper sound, but that version apparently had manual-based copy protection, and also wasn't developed for as long post-release as Ragnarok was.) It also has permadeath, unlike Castle. Perhaps most importantly, because it was a DOS game, it's still simple to run on modern Windows (also unlike Castle).

Great to hear! I just hope he doesn't just stick it on Steam and call it good if/when it comes out.

This seems like a non-sequitur. Do you mean that because few people have mentioned Doom, that must mean that those not mentioning it haven't played it? If so, I would invite you to read the thread title again. ;P

To me, those games are just too modern for me to call them "legacy".

same as with the word "classic", the word "legacy" is a word that has many different meanings and conotations, so what you read into it is comepletly up to the reader.

I thought it's Dragon's Lair Trilogy (1983) in my case, but you've realized me that, indeed, it's probably Ultima (1981).

Might and Magic 2: Gates to Another World from 1988.

My answer is both the oldest I, or my family, own and the first I remember beating which is Contra on the NES.

I call them "legacy" because they're apart of old franchises. I mean to say I've yet to complete Mario Brothers/World/64/Sunshine, Metroid 1/2/Super, or any Zelda before Wind-waker. So Mario Odyssey, Breath of the Wild/Tears of the Kingdom, and Metroid Prime 4 (please exist) are all still "legacy" games.

Was MM2 really that late?

Pong !
But seriously: SSI/Forgotten Realms Series - Pool of Radiance, Hillsfar, Curse of the Azure Bonds, Secrets of the Silver Blades, Pools of Darkness.
There are few RPGs today that let you import characters, start at high level, even though most use adaptive power bot capabilities. I miss that.

Technically, it would be 1989 as I played the DOS version.
However the game was first released in 1988 on Apple II, PC-88, and PC-98.

https://www.mobygames.com/game/1620/might-and-magic-ii-gates-to-another-world/