This feels much more satisfying and well-tuned than the original Sanctum - and the addition of co-op takes it to a whole new difficulty level.

The combination of tower defense and hack-n-slash works fairly well together, though the classes are completely unbalanced. Melee is by far the easiest.

Orcs Must Die! [Steam store page]
While not co-op like the sequel, this game has an excellent sense of humor and decently balanced maps. Too bad the humor didn't carry over to Orcs Must Die! 2

Defense Grid: The Awakening [Steam store page]
A carefully balanced tower defense game with excellent voice acting that weaves an interesting story throughout the campaign.

It looks nice and is enjoyable, but could use some tweaks especially at the more advanced levels. A bunch of enemies in a row can tag team your resources quickly down the line from quite a distance away - which feels a bit cheap at times, especially since the resources travel much faster when rubber-banding to an enemy unit.

It also would be nice to have more control over tower targeting priority, or even just a manual targeting mode. For example, in the later levels you want guns to tear the shields off targets before the lasers attack them - but the guns stay focused on a target until it is completely dead even if they aren't doing effective damage.

It's still one of the better TD games out there though, excellent polish and balance.

Atom Zombie Smasher [Steam store page]
Fairly easy on default, yet nearly impossible to win without using the chooser or combatant modifier - since it often gives you a poor selection of mercenaries. The balance could use a bit of work, since chooser lets you faceroll - but overall fun.

Revenge of the Titans [Steam store page]
A tower defence game that keeps you on your toes. There are tons of upgrades and not enough time to buy all of them - you definitely won't get through all the levels on the first try.

Plants vs. Zombies GOTY Edition [Steam store page]
A fairly relaxing arcade game - while repetitive it can get tough on the endless levels, especially right before each of the endless achievements. Would be nice if the game remembered your choice of plants between levels

Infectonator : Survivors [Steam store page]
The game concept/design is solid - scavenge supplies, rebuild your base, try to rebuild the car to get out of town - but there are a couple fatal flaws that really hurt the gameplay.

The main one is the AI - it's dumb as a box of rocks, and will walk through toxic slime (which kills the character in a few seconds) without hesitation, even when there's an easy path around it. Most of my damage and deaths in this game were due to the AI making stupid decisions.

And when a character "dies", they can be revived for around 15 seconds - which is fine except that it takes like 7 seconds to actually revive them. The timer doesn't stop as you're reviving, which means that if you get to the character with 3 seconds left on their timer, there's no chance you can save them.

It makes it frustrating if you're in the middle of fighting zombies and can't start healing immediately. I'm sure there are upgrades to make heals faster, but it's still a flawed design.

Orcs Must Die! 2 [Steam store page]
They added co-op, but lost some of the humor that made Orcs Must Die! so fun. Similar gameplay, with more expansive levels to challenge two players.

An interesting blend of FPS and tower defense. Seems geared toward multiplayer, and probably fun in that aspect. Novel controls - easy once they're familiar. Flaw with anti-aircraft guns - they all shoot at the same target, needs better AI for that.

Sol Survivor [Steam store page]
Graphically reminiscent of Empire Earth. There is an overload of text in mission and descriptions. The hotkeys are well done. Incredibly easy on "normal" - can't imagine now "easy" must feel.

Anomaly: Warzone Earth [Steam store page]
A tower defense role-reversal where you control the attacking units. It lacks the ability to stop or reverse your units, leaving you to determine the route and do repairs and special abilities. A bit unforgiving, requiring no casualties on normal.