If you like Immortal Defense and would like to spread the word about it and make something doing so, and own a website or blog, you can link to the game and sell it as an affiliate -- which means you'd get a percent of any copy sold that you refer someone to (currently set at 35%).
To do this, you can set up an account on BMT-Micro here.
You can then search through their catalog of games until you see mine, click "Join" and then click "Show links". They then provide you with a link to the demo and a link to the purchase page which you can put on your site. The links should look something like these:
Order Link: "https://secure.bmtmicro.com/servlets/Orders.ShoppingCart?CID=3256&PRODUCTID=32560000&AID=1356080"
Demo Link: "https://secure.bmtmicro.com/servlets/RIP.DemoDownload?PRODUCTID=32560000&AID=1356080"
Except that the ID at the end will be your own ID rather than mine.
After they're on your site, when people download the demo and later decide to buy the game, a percent of the sale price will go into your BMT-Micro affiliate account. They then send you a check monthly.
To see an example of how affiliation works, you can see me affiliating the games of some of my friends here. You can also read more about affiliate marketing on Wikipedia.