I picked this up on sale, and now that I have played it, I would have paid full price for it. I have found it to be a simple and addictive tower defense game that surpassed my expectations. My game play experience is based on playing under Ubuntu 14.04 64-bit with nvidia graphics (mainly) and on Windows 7 Pro 64-bit also with nvidia graphics.
It runs very well in both Linux and Windows, with no noticeable lag or other issues of any kind. I am sure this will play even on older hardware as it has multiple resolution options from 640 x 480 – 1920 x 1080, and 6 graphic options as well ranging from Fastest – Fantastic.
The game has a progressive path, starting out simple and getting a little harder with each level you play. Each level has 3 difficulty settings, Easy-Medium-Hard, which would appear to correlate to how many Hit Points each of the enemy units have. There are 6 different types of Defensive structures that can be built, which can be seen in the games screen shots, and each one can be upgraded 4 times after being placed for money which you earn by killing the enemy units. The building upgrades are progressive in cost, each one doubling in price.
You can pause, speed up, and save the game play as needed. I personally found that saving the game on the Hard difficulty allowed for an easier less and time consuming way to get the Steam achievements.
Some people have had problems getting past levels on the last wave or two. A tip I can suggest that works for me is to sell my structures the enemy unit/units have passed and rebuild further down the path as needed.
I have read some negative reviews about the defensive towers AI not being the best and not targeting units correctly. While I agree they don't always target they way I want them to or think they should, you can predict how they will behave and build accordingly to overcome their shortcomings. I observed this during the first 5 or so maps and then built accordingly for other maps and no longer was bothered by this issue.
If your looking for a simple, fun tower defense challenge then this game is great and definitely worth the purchase!