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I'm playing as Necropolis, and I've slowly made it to the fourth map. I also have the browser open. Naturally, I get distracted often. I have upbeat battle music playing in my headphones. SUDDENLY, some heavy metal or something similar starts playing in the background. Then, just as SUDDENLY, it stops. I thought for a long time whether it was a bug or a feature. Now, I decided to switch back to the game during this musical pause. Turns out it's a feature. Remember the April Fool's video where different units were showing off their skills? The developers put that into the game. I saw with my own eyes vampires playing on their own swords to the corresponding music. There were also some ghouls doing strange movements, but I didn't have time to look closely.
Yes, probably everyone else has noticed this too, but I just had to say it.