
And you thought it was some giant building in Dubai. Nope. According to measurements converted from in-game models ad referenced for scale, City 17's own Citadel, constructed by the Combine, easily takes the cake.
Standing at 8430 feet tall, the Citadel easily, ahem, towers over the competition, Dubai's Burj Khalifa only managing to get 2723 feet off the ground (note: the image above is in metres).
Before anyone complains about the tense used in this post, let's assume there are people here who haven't played Half-Life 2's episodes.
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The scale of Half Life's Citadel, compared to the tallest buildings in the world [Reddit]

DISCUSSION
(note: the image above is in metres).
Well duh, everyone uses metric.
Except for you guys. But hey, no one's perfect.
edit: and by you guys, I mean the states. I'm pretty sure you're the only guys still stuck on Imperial.
edit2: nevermind, it's not just you guys. Metric is used by everyone though, but for some reason it's not the typical standard. How odd.