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Well, this is why I don't make many popular maps.
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Creating a popular map are not meant by recreating something that already been done over and over again such as the TD generic of yours, which lack of interactions and uniqueness.
Making a map great is a very, very difficult matter and not as easy as you think it was. For established genres, lots of that can be cut away by seeing what works and what doesn't in other maps, but you need to seriously think and plan it.
Being fun needs the right balance of lots of different qualities, and that's something that's very hard to achieve, and will always, even for professional game designers, be hit and miss.