I have spent many hours playing Defense style games. On the PC I recommend going through Steam to get your PC games. Steam is best because you don't have to worry about losing your games on disk or getting them scratched to the point where they don't work. If you use Steam you can just download your game again if you have a problem with your PC or if you play at home and at work you can install it at both locations.
My 6 Favorites are:
1. Defense Grid: The awakening, PC Game (Link to the Demo)
Defense Grid is an awesome game that I have spent way too many hours playing. There are several Downloadable Content packs that you can get from steam. The makers of Defense Grid Hidden Path Entertainment just came out with a full-story expansion called Defense Grid – You Monster DLC. The aim of the game is to keep the aliens from getting your cores and leaving with them. If you lose too many cores you lose power and your towers shutdown and you lose the game. The computer game talks to you throughout the game to inspire you or to antagonize you. You place your towers in certain locations around a predefined map to destroy the approaching enemy or to slow then down so they take more damage. There are 10 different tower types with 3 levels of upgrade per tower. The towers can't be destroyed by the enemies, but they can be sold so that you can buy new towers or to rearrange the map to slow down your enemy. There are 15 different types of enemies. Of course certain towers defend better against certain enemies. Some of the enemies have force fields around them which need to be destroyed before the enemy takes on any damage. There are flying enemies which can be destroyed with the missile, canon, and gun towers. I prefer to play the game in a sped up mode which makes it more difficult. You get medals throughout the game by completing certain tasks. You can zoom in and out to get a micro or macro view of the action.
One of the harder maps from Defense Grid:
2. Orcs Must Die!, PC Game (Link to the Demo)
The aim of the game is to stop the Orcs from passing through the rift. If you let too many Orcs through you lose. You are the last of the Wizards and you can keep the Orcs from going through by setting traps for them and by directly defending the rift. As you progress through the game you get more traps. There are 19 or so traps available such as the grinder which sucks enemies into it and grinds them up and spits them out. The traps require some time to reset so that they become viable again. The 9 slots you see at the bottom in this image can be filled with different traps, rings, players to right for you and a second weapon. You buy the traps and upgrades from your coins from killing the Orcs. There are 3 types of rings that you can wear, fire, frost and lightning. There are 2 types game players you can bring into help you an archer and a paladin. Get the right combination of traps, rings and fighters and you can keep the Orcs from the rift. At first you only get certain traps to fight one onslaught of Orcs, but as the game progresses you get flying Orcs, and multiple types of Orcs coming from behind several doors all at once. You can see the Orcs at the top of the screen in the screenshot below. There are also portals in some of the levels to get you around quickly from one side of the map to the other that I recommend highly. There are also Weavers that you get later in the game that will enhance your capabilities. See the screenshot below. I had problems getting my downloadable content from steam on this game. Orcs must die has a few downloadable content packs. Great game and I would recommend it highly.
Orcs Must Die Traps and Enemies Screenshot
Weavers from Orcs Must Die!
Plants vs Zombies for PC, iPad and Android. I have played both iPad and PC versions and they are very similar. I prefer it on the iPad because I can kick back in my chair and play it casually. The aim of the game is to keep the Zombies from eating the brains of the residents of the house. Wave after wave of zombies with different capabilities go up against your plants with different capabilities. You plant your plants to stop the zombies. The zombies come down the lanes and try to get to your residents. Later in the game some of your plants can defend other lanes, but initially they only defend the lane that they are in. If a zombie or bunch of zombies reaches the house you can use the lawnmower one time to kill all of the zombies in that lane. Keep in mind you can only use the lawnmower once per level. Later in the game you can also purchase upgrades for the lawnmower so it goes over water and on rooftops. Zombies try to eat your plants and sometimes they succeed. At the start of the level you get an idea of what types of zombies will be attacking you so you need to plan accordingly by planting the right type of plants to stop them. Just when you think you have it all straightened out it becomes nighttime and you need to plant mushrooms to get coins to purchase more plants. Later in the game you need to contend with the pools in the lanes and the rooftop where you need to plant your plants in flower pots. Try the adventure then try the survival mode. It is a fun casual game that pulls you in.
Plants vs Zombies Opening Screen
Plants vs Zombies Adventure or Survival Screen
4. Field Runners, iPad, Android, Chrome (Demo Link on Chrome)
The tower defense game started on iPhone and now it works on iPhone, iPad, Android, and Chrome as well. You can build towers anywhere on the map so there is no confined path that some other defense games have. You create your own path to stop the marauding enemies. As the ground and air enemies come through your towers destroy them so they don't get through. You get 7 weapons to stop the enemy.
5. Army of Darkness, iPad Game (Link to the Game)
If you have seen the Evil Dead series and especially the movie Army of Darkness you remember Bruce Campbell. “Shop Smart Shop S Smart”, famous quote from the movie. The aim of this game is to stop the evil from getting the Necronomicon book which is the book of the dead. The evil sends various enemies to get the book and you stop them with weapons, tower upgrades and weapon upgrades. Fun game with some quotes from the movie. You get to fight the lady in red from the movie and Bruce Campbell's twin from the movie. Use his boomstick to try to stop them as well as his car.
6. Sentinel 3: Homeworld, iPad Game (Link to the Game on Android)
Great little game where you stop the alien race from getting through your forces. If they break through to your base you lose. You are taking your fight to the alien homeworld and you unleash a huge collection of weapons on them to defend your base. There is an orbital weapon which pretty much devastates any of their forces it touches later on. There are 20 levels. Each level has a commander unit who can level up to get extra abilities. There are 20 turrets, automated drones, and the orbital ship weapon which you can unlock as you proceed through the game. You customize how your weapons are setup before each mission. This is the 3rd installment of the series of this game.
Make sure you checkout the demos and see which ones you like the best.
You might want to checkout the Unstoppable Gorg Defense game when it comes out in 2012.