As of this writing, this game is almost 7 years old. For perspective, it came out only a year after The Orange Box did. When it was first released, it was undeniably the best tower defense game in the world -- but that's not surprising, because its only competition was Desktop Tower Defense and a handful of other shoddy flash games that were starting to ride the trend.
A lot of time has passed since then, and hundreds of weird and interesting tower defense games have been released. So, is Defense Grid still the best? ...Honestly, I have no clue. I haven't played as many tower defense games over the years as I would have liked, and I blame it all on the quality and longevity of this one.
The people who made Defense Grid understand where tower defense came from. Everything about this game is built to look, feel, and sound like retro StarCraft, from the isometric view, to the color choices, to the tower types and damage types. Gun towers are basically bunkers of marines, inferno towers are bunkers of firebats, cannon towers are bunkers of ghosts, and so on. They’ve even borrowed some classic towers from other games, like the tesla coils from the Red Alert series. Even today, it still hits me with that old 2002-era nostalgia of searching for tower defense maps on Battle.net
and that pumped feeling of starting the map with a full lobby of people (because, of course, you needed a full lobby to even have a chance!)
The biggest reason why Defense Grid is so great is because of its scoring system, which rewards even the tiniest improvements in performance with exponentially more money and score. It appeals greatly to the gamer's OCD, since the better you play the beginning of a mission, the more freedom you have to build deadly mazes for the remainder of the mission, the easier victory becomes, and the more often you can see your score skyrocket into the hundreds of millions. Fast-forward and checkpoint-rewinding controls make it simple to re-do every wave until it is perfect. It also creates an ultra-hardcore metagame with a massive curve of leaderboard scores, if you’re into that kind of competition. You’ll need to learn some really annoying advanced techniques if you want to break into the top 100 on any mission, or you could choose to compete in maps and game modes where the techniques don’t apply. Or you could simply choose to compete with yourself and your friends, or just aim for the pre-set gold medals. It’s really up to you.
I've got over 130 hours logged into this, and I'm finally coming down the home stretch of getting all the achievements. God, this game has a lot of content, thanks to over 4 years of continued support and expansion sets. The expansion sets are quite solid, too; at one point I thought I had found a strategy which works on any mission, but then the DLC campaigns came and changed all the rules, and made me rethink everything.
Is Defense Grid still the best? Probably not, and I will have a lot of tower defense games to explore once I’m done with this one. But one thing is for certain: this game is the sole reason why future tower defense games had to start trying new things, because it got the basics about as perfect as they can possibly get… at least for those of us who still occasionally see hordes of zerglings filing through mazes of bunkers in our waking dreams.