Чебурашка
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Conflux is a strong castle, but not overpowered.
Fairies die instantly. Encountering Fairies is a Necromancer's dream.
Strong, unupgraded units of levels 2-3 – how else?
Fire/energy elementals – an average unit considering everything.
Earth elementals – that's just laughable.
Psycho/magical units – the damage is area-of-effect, but they only have 75/80 health and low damage.
Birds and Phoenixes – well, you have to compensate for the drawbacks somehow.
In general, the castle is very dependent on upgrades, even though they are cheap. But they take resources and time.
University – and you don't want to pay the money? It's not overpowered in any way.