Incremental + Reverse tower defense is a great idea with a lot of potential. The game is pretty fun. The current scaling is awful though. The early missions are pretty easy and are done very quickly, but the royal capital stage 10 is an insane jump up in difficulty, especially when there are nearly no more upgrades and you have to wait an hour for the next curse energy upgrade.
The game has potential, but needs some balancing.
I also think there are some ideas that could make this more interesting. Instead of stages, imagine if there was a world map. You'd choose which area to invade first, starting from a beginner village and working your way up to cities, holy lands, and the capital. If the map was large enough, maybe you could have different countries/nations that specialize in different towers like magic/holy/mechanical/tech etc, or maybe even an opposing undead faction. The possibilities are limitless.
There would be rewards for defeating advanced regions like relics that give you bonuses or boost your zombies. Of course these advanced regions would be significantly harder and may contain modifiers that reduce zombie effectiveness. Imagine a holy city having a city wide aura that damages zombies constantly. Choosing to invade surrounding towns and cities will reduce its effectiveness/radius.
A rebirth/prestige system would also be nice.
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I used only ghouls and wait until all your power ups are off cool down full mana. Then have the forward priest and fire towers disabled to make you life easier before it start (Make sure they don't drain your mana until you begin the attack). Then the attack use all the spells then quickly disable all priest and fire tower, nothing else, the timing is tight but there should be a small group able to sneak in and beat the tower in one attack (You do have to have 1 off max cursed energy generation to do this though and if you get max energy generation it's easy) one more edit I found If you want to make your life easier and the timing less tight then you can ignore 2 of the 3 lanes so they don''t take energy to stun