Hello Razwerd,
I just finished playing your map and I have a few comments to help you improve.
There were a number of big issues with the map that I think can only be fixed with time and dedication.
The map seemed to have nothing unique, compelling, or otherwise engaging to keep players interested. The game was very short; creeps seemed to spawn at ridiculous rates for having only 2 human players. The "gate" destruction mechanic does not work well, particularly when there is no way to repair them, and you pretty much lose the perimeter defenses as soon as they die.
The heroes were uninspiring; my Dumbledore hero had four default wc3 abilities. While it's true that his ultimate was useful, it felt like I just clicked an AoE and watched things die, in an otherwise hopeless battle against epic amounts of skeletons.
The AI was bad. Sometimes enemies would pause randomly after killing our towers, and sit there for a good 60 seconds. Our AI was equally appalling. "King Atlas" (sic) rushed the enemy as soon as they got close, and died almost immediately, despite his HP value being relatively high.
The towers had problems also. The cannon towers do not have Projectile Homing enabled, and they would miss their main target all the time. The Arrow Towers were just plain weak. Also, none of the towers were enabled to give XP to nearby heroes. This is a value that can be changed in the Gameplay Constants, and would help heroes gain XP at a more constant rate, rather than having to compete against your towers for XP.
The map was small, and the terrain uninspiring. The few recipes that were there felt like they had been thrown in rather than an integral part of the game. I didn't even know what to buy, but to be honest it wasn't a problem, because in the entirety of the game, I was able to purchase exactly 1 recipe item (before the imba skeleton waves crushed everything).
There was no option to build, repair, or otherwise fortify the castle. Our troops were few, and they would get almost immediately slaughtered. Buff the Heal on Priests and have them autocast it; right now they spend half their time auto attacking skeletons when you or your allies need healing.
Sadly, this game will take a lot of work to fix. If this was an initial attempt at a castle defense map, then my advice is this: Don't give up, but perhaps start over with a new map, new heroes, new items, and new terrain (perhaps even a new concept).
I remember my first maps well. They were honestly not at a level that I could have submitted them to the hive workshop and expected much positive feedback. But that's not to say that you can't improve. Use this map as a lesson from which to learn. You made a lot of gameplay/design mistakes here, but I believe you can come back with a better second attempt, and an even better third attempt, etc.
Unfortunately, I don't believe this map is at a level where it can be played even casually. Consider redesigning large elements of the gameplay to make it more replayable and fun.
Good luck with future maps, and don't take this criticism personally. I believe you can improve from here.
Mech