I still feel Marathon was at equal level, even surpassed it in several areas. And released maybe a couple of months after Doom.
But it was only known by Macintosh gamers at the time.
In which ways I feel Marathon was better:
- better light effects
- ability to look up and down
- secondary effects on various weapons
- deep story (done through interactions with computer terminals)
- variety in physics based on levels and weapons (e.g. grenade launcher) is affected by physics
- having to deal with oxygen depletion in some levels
- some AI that aids you (defense drones that support your fight against the aliens)
- more variety in level objectives (e.g. prevent humans from being killed by aliens)
- more comprehensive multiplayer (I believe Doom supported 4 players; Marathon supported 8 - Marathon has more types of multiplayer games than just Deathmatch).
I think Jason Jones [0] was probably at an equal level as John Carmack at this point in his career.
A deeper comparison between the 2 game engines can be found here: https://www.doomworld.com/forum/topic/78805-marathon-vs-doom...
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[0]: https://en.wikipedia.org/wiki/Jason_Jones_(programmer)