Ok, I can make you a underground arena but it won't be in the center under all the fighting because of wc3 pathing so I'll place the entrance in the center and the arena at the edge of the map. The problem isn't that it's not possible the problem is that having 2 planes over each other maks pathing a problem for units beneath so if I do that you're going to have to find someone that has a solution to that and trust me there ain't none that works properly.
The absence of a terrainer more fairly a terrain is a serious blow to first impression and look of the map. I understand that you want a underground arena but the whole map looks like one more exactly like footman wars!
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This is a story-oriented Extreme Military Formation gameplay.
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Tus I recommend you make some changes! Allow me to make the whole terrain for you, change the map to a large size, should be somewhat dota scale (that would affect your gameplay), Change the scenery to outdoor forest with ruins.
Like so IVD
Maybe you would consider making it a bit more tactical like managing your army first then attacking, the gameplay is too short.
The game lags a litle because of massive unit spawn! That will create a problem for multiplayer.
I won't be doing anything without your approval, what do you say?