Firstly, history isn't my strength.
Celts: Swords, battles, tribes, mud, no dentists, earth, bronze things like cauldrons, stone tablets, no specific religion but pick any which suits, mayhem, people tending to land, chickens in coops. (whether or not any of this is actually typically Celtic, I don't know.) And weirdly when I hear the word 'Celts' I think of England, although I'm fairly sure the Celts were all over Europe, Switzerland esp, but I've forgotten all that. Also, for some reason I'm reminded of the Vikings, but that might be my own historically confused mind. I must say I've forgotten pretty much most things to do with who the Celts are. I think the Celts aren't the usual feature in stories, but I don't know. If so, this might be a good thing as you're working on an open canvas as far as readers are concerned.
I won't mention a little Gaul with a moustache and his fat friend with stiped trousers and a menhir on his back.
Anything before the 18th Century. For me 'ancient' is a umbrella term and I don't see not much other than old stuff. But a medieval image with castles is strongest (porb because I came here first as a 'foreign language tourist' being herded around castles).
Roman ships arriving at the south coast, hoards of soldiers spilling out of the ships, building roads and bridges, 'slapping down' the locals, cobbles and other square stones, burying the dead on the side of the roads and not particularly caring but they can just lie about, postal pigeons, orgies abroad, orgies abroad that go out of hand but who cares cos we can be corrupt and do all sorts being abroad I mean who is going to report it back to Rome. Soldiers leaving home. Bringing modern ways to the locals. Amphorae full of wine. Also when I think of Roman Britain, I can't help but see Rome in my mind's eye, like the octopus with its tentacles.
Not sure this is helpful to you.