Thanks for the replies guys. Lincoln I'll try playing your map a bit today, even though it's emperor. Seraiel I've seen some of your HoF games. Thanks for the detailed feedback. I don't know why this would be a troll thread though haha.
It's true I don't have much experience on CIv4 as I picked up the game two months ago, but I've played Civ 1, 2, and 3 for years. I've won Deity but with only an aggressive/charismatic Praetorian rush on a tight-packed map and dozens of reloads (mad cheating). I've also lost some close space-races in the early 1800s where I had military parity and a land lead, Mining Inc, and the #1 industrial base, but couldn't nuke enough enemies towards the end (nearly 100 launched warheads and several 1-turn military victories).
I also win every single immortal game I play with careless and sloppy play but on deity I regularly get my ass kicked no matter how well I start off. Ie. if I get the GL built and an early tech lead, I get attacked within a few turns.
I had the thought that this thread might be trolling, because I noticed that you're a good player, therefor I thought you might be someone who regularly beats Deity, got himself a new username and now makes fun via asking questions like "why this" when he himself manipulated the map in a way he thought nobody would notice, but if you're honest, it's ok.
I believe that the reason for you not beating Deity btw. is, that you rely on tactics that you should use. If you reload heavily, you will never find out what really works and what doesn't.
Like with Praetorians. Praets i. e. work against an Opponent having Archers (always) , Praets work awesomely with Catapults until Maces, and Praets need Numbers:
-> Learn mechanics how to get more production (mainly the whip, but also have enough Workers that can chop Forrests and don't REX too much, 3-5 cities for a Praet-war is more than enough. Also choose the weak targets, try to get someone into war to then backstabk him, try to make him revolt before attacking so he cannot whip reinforcements, leave cities in the back undefended to lure out defenders and so on) .
(sry for the style on that, but those are really very powerful keys)
That's why I guess I'm trying to read more and ask more, because clearly there are gaping holes in my game. I think it might be poor teching, and not properly utilizing the specialist economy with pacificsm/caste system enough.
The HoF fame games amaze me. I've seen domination wins where the capital didn't even have a barracks built. Or the cultural wins where they just spammed great artists into size 1 cities while keeping 50 spies in their capital and somehow avoiding war on +14 relations with all leaders and like 6 defensive pacts.
Clearly there are game-breaking ways to play and critical diplomatic tactics that are eluding me. Every time I watch a playthrough I learn about exploits like gifting your vassal a dozen warriors.
By the way I was only building those libraries because I traded for pottery on that same turn or just before. Clearly I would've whipped those terraces.
Specialist-Economy with CS is something that helps in Spaceraces and when having REP. Otherwise, Caste is quite weak.
HoF games where the capital didn't have a Barracks, those are "cheese wins" where the map was small and someone rushed everybody with only few troops, "real games" that go to something like 1000+ AD will definately have Barracks in their cities.
Culture wins with Spies don't work via GA-spamming, they work via bombing 3 cities with 1 GA each to have "seed culture" , and then generate about 20k-30k of

to then use the "spread culture-mission" to spy the city to legendary after having gifted it to an opponent, and then reconquer it before that opponent was able to send troops there with little force -> win.
More important though:
1. Gifting Warriors to someone shows that he cannot / doesn't want to win normally -> weak.
2. Not having Granaries at, let's say 1600 BC at latest, shows that you must think about priorities. I have many games where I don't even build Libraries in many cities when going for Conquest, but Granaries, hell, those buildings must really be in every city, as early as available. Libraries one needs either 2 (Capital + GP-farm) or x (number that unlocks Oxford) , and that's it, at least for Conquest games.
All I can give for now.