As I said. An opportunistic attack by the French while GB was occupied with other things (America and India). Only ~3000 men involved. And that's the only significant land battle on the island in the period.
I'm picking this one because the Hundred Years war battles are plentiful if you actually dig into the history.
That conflict ends less than 10 years into the game's period. If you were arguing for Jersey's inclusion into CK2 I'd absolutely agree with you, but this is EUIV. I don't think significance during the HYW is a good argument here.
And jersey is absolutely riddled with Towers and castles. For a measure of fortifications per square mile, it IS the most fortified place in the world (that isn't literally a castle town).
Most of the fortifications are from before or after the EU period though. Elizabeth Castle is the only really notable fortification build during it. Which by EU standards is probably a fort level 3-ish, though you can't really compare because it primarily defended against naval rather than land-based threats. (something else EU doesn't simulate: sea forts). The round towers you mention were built only a couple of years before the end of the period, and most of them incomplete at the time of said significant land battle.
Can you even name this one? There are plenty more ruined castles in Jersey:
http://www.jeron.je/thatwasjersey/castlefiles/AF030big.jpg
That'd be Grosnez. I know because it's about a 15 minute walk from my house

. It's... not a good example of a period-relevant fortification for obvious reasons.
I'm not saying Jersey should never be in EUIV, but that it'd be pointless to include unless its significance could actually be added to the game. Because otherwise it's just a 1 base-tax fish (grain in later dates) province with a needlessly powerful fort that defends nothing of value. If the game were to somehow simulate privateering bases (we were the most successful base for privateers in the British Isles against French shipping in the period) or naval forts, then maybe it'll be time to think about giving Jersey its day.
(I've actually just had an idea for the implementation of naval forts. I'm thinking it could give a Fire bonus to friendly fleets in sea zones that border the province. Should be VERY expensive to build though.)
edit: I totally forgot I made this
http://forum.paradoxplaza.com/forum/showthread.php?734585-Mod-Best-Mod-Ever