It's quite an interesting concept. Basically it's a tower defense game, but you don't just build towers and leave or upgrade them like normal tower defense's. You physically build your towers by dragging different block parts and building different types of towers, so there's a lot of manual work, but it makes a lot of sense! It's a very cool game, and I like the style. Back end graphics could be improved (like resolutions settings and anti-aliasing), but it's free and definitly worth a shot!
It does, however, lack the addictiveness of other tower defense games when your towers aren't being destroyed and waves keep on coming in in large hoards. Neat concept, but it could be a bit better... but it's just a game concept! Give it a shot!