Kluka
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Why upgrade Centaurs against such mobs? It's easier to deal with them with 7 Elves.
Zombies have no chance outside the Necropolis anyway.
Inside the Necropolis, yes, Elves are better.
But against the Conflux (which was a computer opponent), upgrading Centaurs turns out to be very useful due to their greater speed and health compared to Pixies and aerial units.
As a result, if you don't have enough Elves with your heroes, instead of upgrading Elves in the Castle, you can upgrade Centaurs in the Fortress. At the same time, Centaurs are upgraded to level 111 to ensure a speed difference of 8 for the scouts.