Because of the location of my Forbidden Palace, is what I was thinking.
In Conquests, the FP has less effect on corruption in the towns around the FP-town, and much less effect on corruption across your entire empire, than it did in Vanilla.
Broadly speaking, what (all) it does now is to remove most of the corruption (though not to zero) in the town where it's built, and increase the
total number of lower-corruption towns that you can own, but its actual location is now far less impactful: you can't use it to set up 2 fully productive cores any more.
How do I make that happen?
Simplest guaranteed method is to start a 'phony war' with someone you can't easily reach (i.e. who can't easily reach you!), then pay one or more of their neighbours (per-turn!) to fight them on your behalf. Once you've lit the first blue touch-paper, the AI-Civs will tend to start recruiting one another to keep the fireworks going. Alternatively, make yourself a less attractive target by building plenty of attack-units (and few if any defenders!
), so that any of your more aggressive/ tooled-up neighbours will be less inclined to go to war with you, and more inclined to bully a softer target. Which approach might work most effectively depends on the geography — but also on what difficulty level you're playing at here (Regent? Warlord?).
If still Chieftain, you can probably just build a stack of attackers and roll over your neighbours one or two at a time. From what I can see in your screenie, most of your neighbours are even less developed than you are, with only Spears and Warriors visible (and of course, the Carthaginian 2.3.1 NuMercs defending their towns), although presumably Archers are also being built...? Javs can kill Warriors/Archers pretty easily in the open (on attack, less so on defence), and a tall enough stack of Javs will also have a reasonably good chance against Spears in towns -- but you're only 1T from Feudalism, and if you can then acquire some Iron, you'd be better off putting shields into MedInf than Javs.
On which subject, I assume you want to start with the Celts, to secure their Iron before they hook it themselves and start building GSwords (A.D.M. = 3.2.2)...? Otherwise, I can't see why you'd want to take Brennus on first, when Willy controls much more fertile territory? But if your Military Advisor already rates you Average to Strong against both, then why not go after both of them (especially if this is still Warlord or Chieftain)? DoW Willy first (before crossing his borders, if RoP-reputation is important to you!), use Utrecht as an invasion route, then also DoW Brennus, and Settle the coastal Hill 1 SW of the (currently) Celtic Iron. If Brennus hasn't got any Iron elsewhere (can you see it on his trade-screen?), then a line of Javs fortified/sentried along that Hill-/Mountain-range could probably topple most Celtic would-be-invaders, and might even net you some Workers(Celtic) into the bargain.
Stick to fending off Dutch incursions while pummeling the Celts, but once Brennus is pleading for mercy, you can start expanding into the Netherlands, aiming to get to Camulodunum, whose Horses will be very useful for the next war. Because until you get some better/faster attackers, there's no point going after Carthage: Javs aren't going to make much headway against Hanni's NuMercs, so you'll need Maces or Knights instead. (Your gold-pile's pretty healthy, so you could start stockpiling some vWarriors in Barracks-towns while you're getting the Iron hooked, then upgrade them: the 782 gold you have already, will upgrade 8.7 Warriors — and you'll have more than that by the time you need it).