The third one you've put out and yet all you've changed is the interface ... Why don't you actually change the track, not what it looks like i mean the way the creeps go round it? And maybe add more towers? I'm not trying to sound mean but i'm trying to help :)
Is it me or does everyone use this one tutoral and then only edit it slightly? I only know this because i've been using it for like a month or two but i'm changing it loads. You wouldn't realise it was the same thing but yeah, you should put some more time into the next one by changing the track, have some kind of difficulty selection, more tower types, different types of enemys ect. you get my point :)