My first introduction to tower defense as a genre was when the game Crystal Defenders came to Xbox Live. In "tower defense", enemy units attempt to travel from point A to point B, and you must stop them by placing defense units, commonly "towers", along their path. It is very strategic, in that you must decide where and what kind of towers you place to maximize the damage done to the enemy.
I was intrigued by the concept, although that particular game didn't excite me enough to sell me on the game. But when Defense Grid came about, that changed.
In Defense Grid, you have a power plant with two dozen "cores", and enemies come into the field and try to grab a core and escape. In some scenes, the enemies follow a fixed path, and you place your towers along the edges of the path. In others, there are multiple paths connecting large, open spaces, and you place your towers in those spaces to shape the enemies' path.
You have a small arsenal of towers to choose from. There are towers that can only attack at a limited range, towers that can lob volleys at long range (but are useless at short), a tower that can counteract stealth and increase resource retrieval, and one that can emit a pulse and slow down the travel speed of all nearby enemies. Once purchased, a tower can be upgraded up to two times, increasing stats like attack speed, range, and power.
Towers cost resources to produce, which are earned both by destroying enemies and as "interest" on existing unspent resources (so the longer you wait to build towers, the more resources you could have).
The game is extremely simple to play. You can easily finish every level in the campaign with very little effort. However, to get the silver and then the gold medal scores on each level, it takes quite a bit of strategy. It is, to coin a phrase, easy to play, but a challenge to master.
Something I didn't find out until after I had already purchased it, is that the Xbox Live version is actually a port of an existing game for the PC. However, the Xbox Live version includes a few more levels. And, oddly enough, it is priced at only 800 points, or $10 — whereas the retail price of the original PC version was actually around $15. So, wonder of wonders, the Xbox Marketplace's normal price was actually quite a deal compared to the PC version of the game.
I found the gameplay offered at the price to be well worth it. It's certainly been a challenge to complete, just from an achievement perspective; trying to maximize scores and gold-medal all the maps and play variants keep it personally interesting.