Bloons was made in flash, iirc. If anything, the best bet for you is to use Game Maker instead of RPG Maker XP. RPG Maker was obviously intended for RPGs, not TDs. Yes, there have been other types of games made like a Ryden X-kinda thing (very well made, btw) and some people have even done 3D stuff with RPG Maker (though it wasn't a full project), iirc.
However, all these people were well versed in scripting and were able to manipulate RPG Maker to nearly its fullest extent. Unless you have scripting knowledge or at least good eventing skills, I'd recommend you not do a TD.
If you insist on doing a TD, you should work on the system and not focus on any graphics. Also, you're going to want to know scripting. Well. And you need to be good at math and visualization. Because, unless you want the smallest of your missiles to have the same collision size as the biggest of your missiles, it's going to be hard to detect collision.
I don't mean to discourage you in making your TD, but I do mean to inform you that, unless you're well equipped with flexible abilities, your project will not work out. At least start smaller.