Archimonde destroyed Dalaran in WC3 because the old wizard guy wouldn't surrender...
The cinematics are awesome.. Archimonde pulls a voodoo trick on the city. He creates a miniature copy out of sand and whatever he does to the sand-copy happens to the real city.
As far as I've heard, the large arcane bubble (the purple thingy) is a shield created by the wizards to protect Dalaran from strangers. I could've sworn the city was larger in WC3.
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Please "talk up" if your comprehension white-shifts. I will use simple-happy language-words to help you understand.