Not entirely sure when it comes to improvements... I mean, the Ziggurat can be good early on but quickly drops off and becomes a dead tile. Maybe give that some better scaling? Unless, of course, it's meant to fit with Sumeria's theme of "insane early game, intentionally bad late-game" due to them being the first known civilization (historically). In that case, I'd keep it.
I know the Roman Fort is pretty bad, but I'm unsure if it can really be considered a unique tile improvement. Getting this out of the way though, the Roman Fort is bad, but that's because forts as a whole are bad. The thing that really confuses me about the Roman Fort is that- according to the wiki- it doesn't even function like a unique tile improvement.
Quote from the Roman Fort's wiki entry: "It does not count as a unique improvement or grant +4
Era Score when built, nor does it trigger the
Eureka for
Ballistics."
At least for consistency's sake, if you're going to give it a different name, give it the full unique improvement treatment. Give the Roman Fort era score upon building and make it function properly (even if it isn't considered a unique improvement, it has no reason not to trigger the Ballistics Eureka).
And can someone who's played more of India tell me if the Stepwells are any good? They seem a little underwhelming just looking at their stats, but I'd like to know if they could be better in practice.