The skeleton stats are the same as the stats for the skeletons that witches summon in game... why would I change them? This idea is painfully simple, not complicated because witches are a DE troop already present in game.
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I do like that it utilizes fallen enemy troops. I don't like that it uses their gravestones.
Things I have issue with:
1. Gravestones require empty space.
So the tower would have to be place on the outside to do anything. Not a very fun restriction.
2. In my experience, most attacks don't produce that many gravestones.
I usually see around 10-20 gravestones on average. This would mean that IF all the gravestones were in the effective area of the necromancer I would only see 3 waves of skeletons.
3. Disappearance of gravestones.
My thoughts:
Necromancer Tower
It resurrects ghouls from the disappearing ghosts of recently killed enemies. Similar stats as your original idea, but have global effect, resurrects up to 2 every 3 seconds, can only use on screen ghosts, and has a weak projectile.
Reasoning:
Greater flexibility with placement. Limited summoning abilities and summoning resources restrict the overpowered quality of global range. Still "useful" without the ghouls.
What you could do with this is make it a graveyard that a witch resides in. the graveyard would be upgradable to house more dead. when you click on graves the meter fills up and thats what the witch has to raise the next time you are attacked. only four skeletons will be produced for level 1, 6 for level 2, 8 for level 3 and so on and so forth. The skeletons wouldnt be too OP to make a massive difference and theres only one graveyard. Make it so the graveard requires dark elixir to upgrade.
job done
I love the idea because it seems consistent with the rest of the game i.e. all other ranged ground units have their own dedicated defense structure. That being said, why not keep it consistent? The witch tower should act exactly like the witch does now on offense, except the witch tower would have greater range with her staff and spawn an increasing amount of skeletons depending on the level (not depending on fallen troops or tombstones)
Rough example:
Witch's staff attack: 8 tile range
Skeletons: Same attributes as they are now
Lvl 1: 25 damage / max 3 skeletons
Lvl 2: 32 damage / max 4 skeletons
...
Does the graves have to be in the range of the witch tower, that the witch tower can spawn those skeletons?
If no, I think it is too overpowered, a unlurable CC is already deadly, and when you have (endless) troops in defence, it is very difficult to fight.
Supercell add the witch tower it would be epic ;)
So what you're saying is that the tower would sit in the center of a graveyard? Kinda cool, kinda creepy.... It could work, but that might make things a little too complicated, I mean, at this point you are adding two buildings to act as one defense. It might be better to just increase the active range of the tower itself.