I've got a new idea for a Tower Defense style of game.  In the interest of collecting ideas all in one place, and updates, I've created this thread.  Feel free to add to it.

What is this game?
Tower Defense games are amazing little time wasters.  The concept is bad guys move from point A to point B and you (the player) set up defenses to stop them from reaching point B.  Simple enough, but a lot of variations on this theme have popped up.
In this variation, the bad guys are Zombies.  Each night, Zombies crawl up out of ground and make their way towards town hungry for living flesh.  Your job is to place fences and "Heroes" armed with various weapons to keep those Zombies from reaching your precious town's population.  The town rewards you with money for every Zombie killed, which you can spend on improving your defenses before the next wave.

Zombies are cool, but what's special about this game?
All the other Tower Defense games that I've seen have single minded enemies which bee-line directly to their goal, and can be herded easily into a killing field.  Plus, the defenders (usually gun towers) are indestructible.
This game has people as the defenders.  Zombies eat people.  Well, if they aren't gunned down ahead of time they do.  So suddenly, you've got a vulnerability in your defense.  And since the Zombies like any living flesh, they'll detour towards your Heroes as a tasty snack on the way to town.
Also, Zombies will attack each other if they've got nothing better to do.  They will hunt the weakest one down as it flees the stronger ones.

Okay, but I could just fence off my Heroes.  Problem solved.
Yeah, I mentioned fences didn't I?  In the initial level(s), fences act as obstacles for the Zombies, so yeah, cost of fencing is your only limitation.  However, Zombies very quickly learn that they can bash fences.  You can get increasingly difficult fences, but ultimately they just slow the Zombies down to varying degrees if they want to get through.

What kinds of Heroes can I buy?
Currently, we've come up with the following types:
Machete Hero - High Damage, Fast hit rate, Extremely short range, very cheap
Machine Gun Hero - Low Damage, Very Fast hit rate, Medium Range, cheap
Shotgun Hero - Very High Damage, Slow hit rate, Short Range, not cheap
Flame thrower Hero - High Constant Damage, Short Range, not cheap
Rocket Launcher Hero - Very High Damage, Very Slow hit rate, Long Range, expensive

Veteran - After xx number of kills, any Hero becomes Veteran and does bonus damage

What kinds of Fence can I buy?
Currently, we've come up with the following types:
Picket Fence - Weak, Cheap
Metal Fence - Strong, Not Cheap
Electric Fence - Weak, Not Cheap, Damages Zombies
Brick Wall - Very Strong, Expensive

Also, damaged fences stay damaged each level unless you spend money to repair them.  Your Heroes at least heal fully each day.

What are the levels?
Although the game itself will announce each level before you start buying defenses, here's a run down of the levels thus far:
1) Zombies!
2) More Zombies!
3) Tougher Zombies!
4) Fearless Zombies! (Kamikaze, no sense of self preservation)
5) Boss Zombie!

Every 5th level will be some sort of "Boss" level.  Beyond that is still up for debate.  Here are some ideas we've thought up:
Exploding Zombies!  (Explode on death)
Re-animating Zombies!  (Rise again after first death)
Zombie Heroes!  (They shoot back!)
Phoenix Zombies!  (Flames heal them!)
Limited Ammo!  (Heroes resort to Machete once out of Ammo)

Of course any combination of any or all of these ideas are applicable as harder levels as well.

How do I win this game?
Ideally, you can't.  The final levels should be downright impossible.  It's all about getting the high score.

How is score calculated?
I've seen other Tower Defense games score based on left-over money between waves.  That adds the challenge of being as frugal as possible without dying.  I figure that's probably how it'll be done here too, unless someone suggests a better idea.

Where can I play it?
http://darkshade.homeip.net/zombies will always be updated with the latest work.  Read through this thread to see what's new in development.

I dunno if this over complicates things but I had a suggestion for enhancing the game that involves payment between levels and how you would spend your money.

As you stated before, payment is based on how many zombies you kill, but I think there should be a multiplier, based on surviving population (think of it as survival tax for the populace).  So, for example, in level one you have a population of 100 and you're told you will get $5 per zombie killed.  It is level one so you kill the 10 zombies who attack and that means you earn: $5 x 10 zombies x 100 survivors = $5,000!  

But here is where I think I have an interesting suggestion - every day (the time between levels) that you invest in the town's defenses, makes your town more appealing.  The more appealing your town, the more people WANT to live in it.  If you spend some of your hard earned money on scouring the countryside, you will bring back new residents.  But they won't make it back until the FOLLOWING day.  This way you can spend money to build your population, but do it too soon in the game and you'll only draw in small numbers (if your town's defenses are 3 picket fences and you only have 4 machete heroes, very few people will be interested in moving in.. but after 5 or 6 levels, when you have your first brick walls and numerous heroes with guns, perhaps 10 or 15 people will want to move in.  

And the more people you have living there, the more taxes you can collect and the more heroes and defenses you can build, and the more people you can attract down the road.  The problem is, it takes a day to find these new people and bring them back, so if you're coming up to a hard level, you might want to bring new people how ASAP!

Of course this DOES raise the issue, how do you LOSE population.. is it every zombie who gets through kills one person?  Is it the same regardless of what type of zombie gets through?

I figure money left over will wind up being the ultimate judge of success, since you theoretically continual to play until your town's population is decimated but what about adding another feature to invest in the town.  Which in turn inve

Looks like your post got truncated Shabadu, but it is an interesting idea.  It reminds me of existing games where you can manically buy extra "lives" if a ton of baddies is going to pour through your defenses.

Yes, the idea was a single Zombie kills a single civilian.  Guess their hunger would be sated by that point.  That means if you start with 100 townsfolk, you can only ever let 100 Zombies through before game over.

It is a strategic benefit to be able to encourage more people into your town.  I don't know that it should "naturally" grow as defenses increase, since the whole point of tougher defenses is to counteract the tougher enemies.  But I can understand wanting to essentially "buy" more townsfolk between waves.  I think that's reasonable, and can be explained by diverting money from defenses towards feeding new refugees.

First test, creating little autonomous Zombies.  Unfortunately, the current code is waaay too slow.  Get about 50 Zombies on the screen and it begins to crawl on my mac mini.  I want to be able to have a hundred of these guys, plus heroes and flying bullets.
I'll try again with some high performance sprite routines I've found on the web.
For now, you can see floaty Zombies at the permanent url:http://darkshade.homeip.net/zombies

I noticed definite slowing down once I had spawned 3 zombies, then 7 zombies, then 14 zombies and I stopped counting after that on my work laptop.

22 Zombies on my laptop becomes significantly slower.  At about 10 it becomes noticeable.  Of course, you can detect a slight slowdown with each one you add.

One of the sprite frameworks I'm going to investigate demos over 3000 objects moving around the screen.. on the mac mini!

Zombie Test 2: 100 screaming fast Zombies floating around the screen.  I get a steady 24 frames per second.  Let me know if you get less.

As usual, it's at http://darkshade.homeip.net/zombies

My work laptop tells me it is 24 fps too... they all seem to be boogeying along pretty quickly.. much faster than 14 were on the previous test.

Here's Zombie Test 3.  Now there's a couple soldiers who target the nearest wandering Zombie, and a few crates to just get in the way.  Also everything spins to face the direction it's heading.  See how that runs, as it dramatically effects the number of zombies I can have on the screen at once.

As always: http://darkshade.homeip.net/zombies

Zombie Test #4: Heroes now shoot and kill Zombies.  For each Zombie that gets to the left side of the screen, your population goes down by one.

Zombie Test #4 update: I'm now taking advantage of the health system I incorporated last night.  Instead of a single bullet killing each Zombie, it now just does damage.  If enough damage is done, THEN the Zombie dies.

In this test, that happens to be only 2 shots to kill a Zombie, but it means we can have different strengths of zombie and weapons now.  These zombies have 10 health, and bullets cause 5 damage.

Here's the debug log from one run.  You can see one Zombie gets through and eats a civilian:
Blam!
Blam!
10-Zombie: health = 5
10-Zombie: Dies
Blam!
Blam!
11-Zombie: health = 5
7-Zombie: health = 5
Blam!
7-Zombie: Dies
Blam!
6-Zombie: health = 5
Blam!
6-Zombie: Dies
Blam!
8-Zombie: health = 5
Blam!
8-Zombie: Dies
Blam!
Blam!
Blam!
13-Zombie: health = 5
14-Zombie: health = 5
Blam!
13-Zombie: Dies
14-Zombie: Dies
Blam!
15-Zombie: health = 5
Blam!
15-Zombie: Dies
Blam!
12-Zombie: health = 5
Blam!
Blam!
Blam!
Blam!
Blam!
Blam!
Blam!
Blam!
11-Zombie: Dies
Zombie eats a civilian!
Blam!
9-Zombie: health = 5
Blam!
9-Zombie: Dies

Zombie Test #5: It's playable!  Needs balancing, more levels, more heroes, more zombies, and more barriers.  But it's a start!

Known Bugs:
- In the "Unbelievable" level, Zombies are running backwards.
- Unit placement is off visually.  Note, when the level starts, your units ARE in the exact spot you selected.  it's just the "placeholder" images aren't lined up right.

I notice the zombies get caught up on the walls pretty easily - any way to smooth that out a bit?  But it is looking pretty good otherwise.. will have to test more once I'm not working

Noticed a couple more things before I closed off the window...
#1 zombies just run by the heroes without attacking them (I presume heroes and walls don't take damage at this point though but thought I'd point it out)
#2 heroes don't attack the zombies closest to the settlement.. as the wave of zombies PASSED the heroes they turned to attack the ones closer to the right of the screen instead of continuing the attack on the ones that were almost dead but had run past the heroes current location (but were still within their firing circle).
#3 perhaps a conscious decision but you cannot buy on the fly (as in purchase more walls/heroes during the night attack)... I figured you could

1) Yes, in the published version Zombies don't attack the heroes yet.  However, the current development version has them chewing away on heroes.  "Unbelievable" is downright impossible since not only do the Zombies chew the heroes, they actually turn and fight when fired upon!  Not sure if I want to leave it that way or not...  hence, still in development.

2) Yeah, that was my first stab at a firing solution.  It's "good enough" but doesn't necessarily always pick the truly closest zombie to shoot at.  Also, doesn't predict Zombie position well, so often shoots behind as they pass above or below.

3) No buying during the attack.  You have to sit back and watch.  If you're screwed, you'll have to plan better next time.  I *might* allow buying civilians mid-attack, but nothing else.  (Yes, there are technical reasons for this, and I specifically don't want to give any advantage to "fast clickers")

The crates will be going bye-bye once I get some fences that can be smashed.  Not sure if I'll bother with health meters for fencing.  I think with the first hit they appear damaged, then stay that way until utterly destroyed.