Indeed it is – first of because most of the instructions are in Chinese (or Japanese, I have no idea). This means that you’re mostly on your own and you’ll discover “what does this button do?” on the go, just as it would happen during a Martian invasion. Second, because the little flash aliens in Mars TD are some real tough guys and you’ll need to develop a real, complete tactic if you want to have a chance battling them.
Now for the serious part, Mars TD is indeed a quality tower defense flash game which, even though it doesn’t have some awesome graphics (read: the visuals suck), can still prove to be a great alternative to your regular Tower Defense games since it delivers non stop action and you indeed have to plan all your moves if you want your base or whatever that laboratory is to survive.
So give Mars TD a try and you won’t regret it!