I think that there should be another unit, but not the defence obelisk. That's kind of lame. It should have a really cool name and have a land-air attack fields. It should also require some upgrade made by the Archmage Tower, or maybe just the Archmage Tower itself. I think I like the idea of just requiring the Archmage Tower. This is similar to the Tower of Souls requiring the Library. Also, then you wouldn't need to make a new upgrade and an image and also think of a name for the upgrade. The only thing is, it shouldn't be a defense obelisk because that's pretty dumb. It's not very Magic-like and it has more of a Tech-like name.