I used to agree that terrain wouldn't really matter for a TD. However, with all the terrain-cheap TDs out there, it would be better to give it some attention. Your TD needs to be unique and aesthetically beautiful if it is to be successful. Either people want to acknowledge it or not, funny maps with a lacking terrain these days do not have success for more than 3 months after their approval. This is what I noticed as a map reviewer and, especially, as a map moderator. This is also a major reason why game developers always focus on visuals before anything else. The era of "poor visuals but good gameplay" is over.