I had a problem with it, namely the towers would expire or time out or something.  I could never have more than three at a time because by the time the third gets built the first one decides to pack it in and collapses all on its own.  I really hoped to be able to have five up at once, for instance to build a mini-base at one of the flags on the mines map or something.  But they just wouldn't stick around.

I've been thinking about mentioning that too. Right now, it's just, just possible to have 4 towers simultaneously using Power of the Tower III. That's assuming you build each as soon as possible, and even then you could only have all four for a second or less. By my math, if you could reduce the Rook's casting time by 25%, he could have five at once for that long. Again, that's with essentially zero time with all of them functioning. For the level two ability, it's a joke; only with more than a 60% reduction in casting time could all five be in play. I agree, I think the time they exist needs to be extended. Such a short life also makes it very difficult to construct useful chains linking new towers and the Rook to the main base; by the time the last element is in place the first is already failing.

If they stayed permanently wouldn't you be able to build a hub of them behind some other towers to increase the charge up?

they should vanish after some time imo, however it should be longer than 45 seconds, something between 80 and 100 seconds I think, if they turn out too strong then, damage and hp can always be adjusted.