A respite for the English crown
110 B.C.: The English monarchy offers all the gold in its coffers and some of its finest furs as ransom for peace. The Celts accept, as their economy is still bleeding coin and the plunder from Nottingham will not last forever.
If I wanted to, I could just keep rolling. My Swedish allies have depleted any relief forces that might have stopped me, and York is just a couple turns away from being in my hands. But at the moment, I have more to gain from the nearly 200 gold (literally every penny they have) and luxury resources I'm being offered. Peace Treaties have a mandatory length of 10 turns and, let's face it, I plan to attack anyway as soon as it expires.
80 B.C.: 30 years after the war's end, the English crown requests an exchange of embassies. The Celts refuse, proclaiming that to do such would be to recognize the legitimacy of their faithless rule over the realms of England, which rightfully belong to the holy PC.
Later that year, Sweden, seemingly bent on declaring war on everyone at least once, proposes a joint war with Russia. The Celts point out that they currently have the occupation of England and barbarian raids going on, and Russia is kind of at the other end of the freaking continent.
50 B.C.: Celtic engineers produce the first catapult, which is immediately sent North to the English front.