As it stands, the Black Tower and the White Tower have these benefits: +100% , +1 free specialist, and can turn 1 citizen into sage. Now, they need somethin to set them apart. So, let's look at the Theme for both of them. The Black Tower is obviously sposed to be an evil Tower. And the White Tower is likewise good. Then both are, obviously, sposed to be magic as they both require the "Magical Philosophy" tech. So, we have an evil tower of magic and a good tower of magic. But they need somethin else, somethin more to set them apart. Some bonus, an obviously evil bonus that might make evil civs wanna build the Black Tower. Then an offsettin good bonus that might make good civs wanna build the White Tower. I haven't thought of any two bonuses like I described but I'm sure I will later and I'd be happy to hear anyone's thoughts on it. Then, off a slightly off-note (but no less important one), the Black Tower needs new unique graphics, same as many other wonders currently do. I can experiment a lil and do that tho.