I agree with you, my Unsullied numbers did seem high. Arya Stark not today Game Of Thrones shirt caught that the artillery was placed in front of the Unsullied formations in the preview, but that seems like such a silly spot to stick catapults that I assumed those shots had to be while the Unsullied were still getting into place. Otherwise, I think you make a very valid and plausible point. That said, in Maureen, the Unsullied on the show really only engage against poorly armed and untrained insurgents.
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And Arya Stark not today Game Of Thrones shirt is very lightly defended by inferior Lannister soldiers, so while it seems unlikely, I don’t think it’s impossible that they’ve only lost 500 soldiers to date – especially if the Dothraki vanguard ate the brunt of the casualties at Casterly Rock and the Field of Fire. It looks brutal but short: Grey Worm clearly questions how easy it was compared to what they expected. For comparison, the British Army lost 500 dead out of 10,500 (around 5%) when assaulting Ciudad Rodrigo in 1812, and that involved storming a properly manned fortress in the face of proper artillery (canister shot, etc) and firearms, rather than crossbows and swords.
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Shortly afterward, in the assault of Badajoz, the Arya Stark not today Game Of Thrones shirt lost around 2000 dead out of 27,000, or about 7.5%. And that was a VERY hard fought assault. With the Unsullied having more men than that, no artillery/firearms and the assault apparently being over fairly quickly. I don’t think it’s unreasonable to suggest that the Unsullied had casualty figures significantly lower than 5%. It warms my history-loving heart to see Peninsular War references thrown down on an ASOIAF subreddit