Good Game
Fixes to consider:
-Put a background behind some text, it can be hard to read.
-Make the refill button easier to click.
-Typo fix: In the upgrade menu it says energy instead of gold.
-Make the archers stop shooting when there are no targets on screen.
Potential
Archer sound needs fixed, I had 4, and killed them all, yet the sound of them firing remained and hasn't stopped for the better part of 10 minutes. Even on hard difficulty all you need is about 4-6 archers well upgraded with double shot to win. Also, it allows you to stack merchants, but it doesn't give you a stack bonus of gold. So if you only get one merchant's worth of gold bonus you should make it to where we can only buy one. Also, the Samurai is a little OP. I got four of them working right now with first strike and well upgraded and they're one shotting all the zombies. The sumo guy is useless unless you upgrade his health a lot and damage, then you can just drag him all over at the zombies and he'll attack while you do so. Also, it's been mentioned, but gold is WAY too easy to obtain. In the time I typed this out, I gained well over 30k gold that helped me fund my OP Samurais and merchants. Not a bad game, but it needs some work.
Great Potential, needs more work.
Bug I noticed: when you click on a unit, it causes the cost to go up, even if you didn't place a unit. This means that your costs can quickly skyrocket.
Also, you're already earning money during the info screen. I went to the restroom while this was loading and ended up with way too much cash.
Looks like it could be very fun when you get all the bugs worked out. See if you can't work on the art for the walls and such.
Work in progress
Good potential, but there are some things that really need to be fixed. For example, if you try to place a mystical ninja down at the beginning, it won't let you place any units down. Also, when you restart the game, the price of the unit needs to reset. and when you don't use a unit, the price shouldn't increase. I would like to say that I definitely like the units and the game design. So I am going to rate it a 6 for now, but look forward to seeing the changes made.