I think I should start with the negatives since there's not much to talk about for this one. Jaguar Man was an OK character who has some comedic moments, particularly with her banters with Quetzalcoatl, but I feel like her character was rather underutilized. She mainly served as a plot device to stop a certain forces of nature and that was it. Her cat-puns also got a bit annoying after a while, though I think the anime adaptation might help improve with that if Taiga's VA voices them in a good way (since Jaguar Man is pseudo-servant Taiga after all). Benkei also didn't do much other than acting as Ushiwakmaru's retainer and serving as a plot device to stop corrupted Ushi once and for all, but his character send-off was nice.
The pacing of the story also starts out pretty slow. About the first third of the story is mainly Gilgamesh assigning Fujimaru and Mash to do some side quests across Uruk to win his trust. Some of the quests do drag out after a bit. The only noteworthy thing that happened during the slow part of the story was Enkidu's betrayal (and even that felt rather obvious) I'd say the story didn't pick up until the scene when Gorgon first attacks, which is after the first third of the plot.
With those issues aside, let's talk about the positives, as there's a lot to say for this one.
While the menial tasks Gilgamesh assigns to our heroes in the first half drag down the pacing, it's also through these tasks we got lots of world building regarding the city of Uruk. In fact, I'd say that Babylonia does world-building the best out of all the Singularities. You really get a sense of what life is like in Uruk and the remaining Babylonian cities, and you also get some good insight into what the environment is like surrounding them. I really felt like I was actually in the Singularity compared to the others.
The amount of worldbuilding placed into Babylonia only the made the apocalyptic climax all the more dramatic and emotional, as you really feel the sense of cataclysm and doom when Tiamat and the Lahmu begin their attack. In fact, this dark tone is delivered
really well during the climax. Mash gets a new facial expression this Singularity where she has a really desperate and panicked look on her face that shows her sense of despair (I don't have a picture of it, but if you watch a Let's Play of Babylonia or experience the story from the game itself, I think you'll know which one I'm talking about), and the twist of Tiamat beginning her attack from the Persian Gulf and not from the North really brought the tension since it was something no one expected. Let's also not forgot that Tiamat's true form was quite disturbing, especially when the final fight against her zooms in on her head. The countdown towards Tiamat's awakening done at the end of each day was also a very nice touch, giving the sense of a doomsday clock.
And the Lahmu really do come across as terrifying beasts not just in their appearance (I seriously got chills when I saw how they looked the first time with human lips for their heads, their screeching sounds, and their odd-look bodies), but in the way they acted: they target only humans, tear them apart in vicious ways when killing them (the one scene where the female Uruk citizen screaming how her body can't be stretched like that during their attack on her did a good job portraying it), gradually develop sentience to become more human-like, and eventually admit they enjoy killing humans and toying them in some pretty sick ways (such as that one scene where Babylonian citizens kill each other to try and appease them, but the Lahmu still kill those that did the sacrificial killing anyway).
Tiamat was also a well done force-of-nature villain too. The writing does a good job explaining how she was one of the founding gods of Ancient Mesopotamia and how she went from someone who loved her children to eventually becoming spiteful to them when they forgot about her when creating humanity. When she attacks Babylonia, you get a very good sense of how omnipotent, terrifying, and practically unstoppable that she is with her size and powers. Honestly, I think Intelligent Systems can learn from Tiamat in regards to writing good force-of-nature villains after the disaster that was Surtr, as Tiamat is what I consider to be a well-done force-of-nature villain.
The character writing was also top notch throughout it. Other than Jaguar Man and Benkei (who I'm indifferent to), there wasn't any character I disliked. There were some characters that I initially disliked, but they changed my mind in some good ways.
To start with the star of the Singularity himself: Gilgamesh. I absolutely despise Gilgamesh in Fate/Stay Night and Fate/Zero (namely because of how arrogant he comes across, what he did to Illya in UBW, and how he treats Saber in the ending of Fate/Zero), but this version of Gilgamesh turned out to be a pretty cool dude. He still has many of the same traits that make Gilgamesh what he is: arrogant, proud, and saying "mongrels" a lot. However, this version of Gilgamesh is a lot more noble and kingly. As it turns out, this is Gilgamesh after feeling the impact of the death of his dear friend Enkidu, so he's become more reformed. He also comes across as a sociable guy, and the way he develops into that during Babylonia's story was pretty nice. The scene when he casually drops by Fujimaru and Mash's house to give the next task directly to them instead of having Siduri do it was surprising and nice, and it does a good job of showing how he grows to trust them. He also speaks some wise words to our heroes during the climatic chapters, namely to talk people like Mash out of their fear of the apocalypse.
For some of the supporting heroes, Ishtar definitely had the best development out of all the characters. She initially starts out as someone who doesn't like humans and is rather spiteful towards our heroes in the first few encounters, but the way how they convince her to fight against Tiamat was good stuff. Her character was also quite enjoyable: she's still the Rin Tohsaka we all know and love with her tsunderic-traits and love for jewelry, but she's also quite a klutz. Her quarrels with Gilgamesh were also fun to watch, and her willingness to take initiative during the climax was some good stuff. Ana had a decent character arc where she initially starts out as a very withdrawn person but gradually warms up to others as she spends time in Uruk, with her encounter with the old lady that ran the flower shop being a nice culmination of her character arc. I wish they delved a little more into her relationship with Tiamat-Gorgon just to amp up the personal connection between the two characters though (particularly during the entire scene when Tiamat-Gorgon awakens where Ana could've had a line or two along the lines of "She is my greatest enemy, and I need to defeat her."). Siduri was also a nice side character who bonded quite well with Fujimaru and Mash, and her death (i think she was turned into a Lahmu? I don't completely recall her demise) definitely had some impact. Ushiwakamaru was just as fun of character as always who also displayed the right kind of seriousness when the situation called for it. The way she called out Kingu as she was captured was gold. Her corruption into her dark version version was tragic, and it only added to the dark tone delivered well in Babylonia's story. Sparatcus was a also a great side character with a nice personality and some good dialogue. His words to Mash as he explained to her the story of the Last Stand of 300 to teach her not to give up despite the odds was very memorable.
The Three Goddess Alliance also turned out to be a nice bunch. I was initially disappointed we didn't see much scenes of them talking with each other (as seeing villains interacting with other villains is something I really like in stories, and it was one of Camelot's strongest points in its story too), but I understood why once it was revealed as to why they created the Alliance: to save humanity from Tiamat's destruction, but in more anti-heroic ways. Gorgon is the only one who was the most antagonistic out of all them, but we eventually see her reformed (still retaining a grudge against humans though) once she and Ana merge and fight back against Tiamat. Ereshikigal was rather fun with her Rin Tohsaka personality and desire to save humanity by keeping their souls trapped in the Underworld from the real threat. Quatzalcoatl ended up being my favorite of the three: not only does she have the most screentime out of the three, but she turned out to very noble character. She wanted to strengthen humanity for the inevitable danger, and she has a fun personality too. I did find her love for Fujimaru a bit unnecessary though (but thankfully, she never reaches Camilla-territory with her "big sister" attitude mixed with her love for Fujimaru. I look forward to seeing her animated, especially when they show off her luchador moves and her serpent that makes her a Rider class servant.
Kingu was a good villain. I initially loathed this guy when he was introduced, but in a good way. He comes across as a mean and spiteful entity who wants to appease his master/mother Tiamat, and everything he does makes you want this guy dead (and he even almost died too during the climax). However, once he sees how the Lahmu behaved not in the way he wanted and ultimately concluding Tiamat doesn't listen to him at all, he ends up having a nice redemption arc where he feels useless only to have Gilgamesh step in and give a few words.
The First Hassan also makes a comeback in this Singularity, and it was delivered really well. The twist that the hooded man Fujimaru kept writing into turning out to be him was epic!
But I can't forget my favorite character in this Singularity: the one and only Merlin!
Everything that came out of his mouth was a pleasure to read. Nice backstory, nice personality, and overall just a nice and enjoyable character I can get behind. He is well aware of the misdeeds he has caused, and other characters call him out on it. Despite being OP, he still makes mistakes that impacts our heroes. However, he still cares about humanity, particularly "the stories that they weave" as he likes to call it, and as such, he does what he can to help them out. His voice of reason to our heroes was great, and so was his more casual talk. The banters that he has with Fou were also pretty humorous. Let's also not forget that he managed to summon the First Hassan for that epic scene near the end of the story! He also has some bonus points for a having a character design that strongly resembles N Harmonia, one of my favorite characters from Pokemon.
I mainly knew Merlin as "the servant that broke Fate/GO for his OP-ness," his involvement with the whole Mordred-Artoria fiasco, and his random deux-ex-machine moment during that once scene in America's story, but after beating Babylonia, I can honestly say that I like a lot as a character, and that is added to another reason I want to summon him in Fate/GO!
After reading all this, I think you know well enough what I think of Babylonia. It's up there with Camelot for having one of the best stories out of all the Fate/Grand Order stories, one that I consider top tier!