Commander Keen 4 (ergo my lame pun...

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A childhood favourite for me. That's the view when you're out roaming the world between levels. The levels are classic side-scroller, but with the option to jump, shoot (ray gun stun), pogo stick (big jump but slowed the direction input), grab onto ledges and haul yourself up, and somewhat steer direction while in the air - it's very playable compared to other games of the era. I got the "complete" (not actually complete) playable set off GoG (Good old Games). Keen 1-3 have different games mechanics and are a lot more stuffy to play, but Keen 4, 5 and 6 were all matching in controls/playability but with evolving stories in different settings. Some of the best uses of Midi backing music in any early games IMO too (and the music changes in most levels to set/match the mood). Plus, a secret/hidden language, secret/hidden tunnels, some platforms you can only stand/bounce on for a moment before they disappear forever, etc... there's a lot of variation throughout the levels.
This is a moderate level, made to look easy as the player doesn't die in one of the heap of different ways you can die (don't touch the yellow slugs, don't stand in their slimy poop, don't touch the lava, don't let the bat touch you while in flight, watch out for the little green dots thats expand when you're next to them and bite/kill you, etc... But there's a bunch of different enemies across other levels. 4, 5 and 6 are definitely worth a shot if you like retro games (5 has the best music IIRC), but skip 1-3 IMO as they're a bit wooden to play and lack the secret passages and manoeuvrability of the Keen himself.
[EDIT: If you're going to give it a play, I recommend juggling the standard control layout so Shoot = Ctrl, Jump = Alt, and Pogo = Spacebar. Personal preference though...

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