I think what I liked the most about Immortal Defense was, that you are an important part of the defense. Liek Misery already said, a lot of TD games work that way that you cheese you build stuff and wait for enemies to die. While not everyone may have the same distaste for that kind of gameplay as he has, Immortal Defense puts the player on the active role of playing. Simply because, you, the player are the strongest "tower" in the game. Not that there are any real towers int he game, just points.
The mouse cursor in this game represents your focused willpower (you have no body, you are just a mind with endless power), so you focus your rage into one point and it materializes as bullets into the real world. Sounds more confusing than it is, basically your cursor shoots bullets wherever you direct it. This works perfectly in the game since you have autoaim. Simply move it close to enemie and it fires directly at them.
The points are different aspects of your psyche, they represent your emotions and thoughts that you have while being in space. Points work similiar to towers like in other TD games but not quite the same way. First, you have only a limited amount of them per level. It's not liek yu can build the ones you want as long as you have moeny. Like I said before, they represent your emotions and because of this you can only build them if you HAVE those emotions. So you get a limit of them each level, depending on the current level. However, these points produce MORE points over time. I never fully understood what exactly the unlock requirements for new points were but I think if your towers produce enough damage (kills, they create a new point for you to place. however, if you have a specific point not aviable in that level you won't get it later of course. They only duplicate.
There is another difference to normal towers. Your points react to yourself (since they are parts of your mind), if you charge your special attack, they are drawn to your cursor, if you release it, they fly into the direction of the special attack. There is also a special point that always attacks the same target as you do.
And don't get me started on the more crazy towers/points, the game simply does not want to behave like other TD games. They literally break the genre with this game.