idk what to exactly put there, moat is still an obstacle even in modern context, but assault on a castle with a moat using modern weaponry would be hilariously one-sided. you can suppress defenders with something, use a bridge layer to get inside the moat, then let combat engineers do their shenanigans to "open" castle one way or another. or you can use helis to do the same, or you can just level it all with artillery or airstrike or maybe even loads of ATGMs

that said it's not completely useless. moats but dry were used as a part of fixed fortifications in ww1 quite successfully. freshly invented electrified barbed wire fence and machine guns made them quite hard to pass, especially if you are, say, a peasant from tula oblast born in 1898 that has never seen powerline before. i think the last proper moat use in large-scale warfare happened during iran-iraq war, in battle of the marshes, when iraqis flooded previously dry area known as fish lake and put underwater coils of barbed wire and high-voltage cables. defensive tactic used there was to shoot at assaulting iranians to make them abandon or fall out of their boats or amphibious vehicles, then when they were in the water high voltage lines were energized. iranians eventually crossed the marshes entirely using speedboats. maybe it's not that outdated considering that last recored bayonet charge happened in 2004 (by brits in iraq). ymmv