Scratch

A very simple one would have a sprite for each tower, and a sprite for each enemy. The tower would pick a random enemy (doable with a list and the "point towards" command), and then broadcast something to a bullet sprite, that would then move until it reaches an enemy or goes outside its range, then go back to the tower and hide. The enemy would have a preprogrammed path and if it touches a bullet, it gets damaged. I have a simple TD uploaded on this account.

Jodymoses wrote:

???

He just explained it. Here, let me make it simpler.

1) Make a sprite for each tower, and a sprite for each enemy.
2) Use the block that's called point to, and make a list of all the enemy names
3) Make a broadcast to shoot
4) Make the bullet sprite move towards the enemy, then disappear when out of the range of the tower or on the edge of the screen
5) Enemy gets damaged when touching bullet
6) Enemy health is a variable
7) zammer990 has a simple Tower Defense game on his account.

Jodymoses wrote:

???

What did you mean all you say is "???" Why?

Advanced use of variables and broadcasting is the easiest idea I have, sorry. 
But the whole above posts do make sense, if the above posts confuse you too much, try something simpler?
I'm being of no help, sorry. 

Pick a single element, and think about how to script it, then build a prototype script, if it works, then pick another thing that needs doing. One script you'd need is one to find the distance for all the enemies (harder when you're doing a single sprite for more than 1 enemy, but that's more advanced scripting), adds them to a list, then picks the closest/a random one, and points the turret towards it.