It's an okay setup. Pretty basic. Lots of things bug me, though.
First of all, there's no range indicator, and there's no enemy info. This makes strategy very difficult. It makes it hard to place solid towers or to adapt to a wave. And it seems really buggy. For example, several of my towers' stats were changed when I upgraded other towers. Didn't make much sense unless there was some kind of AoE effect (but there was no indication of one). So it made tower placement even more difficult because the stats were basically randomized once I started upgrading towers.
I'm guessing glitch. Which brings me to a question, mostly. There got to a point where I was fully upgraded on towers and had about 25 mercenaries covering the map, and enemies were still occasionally getting through. Most enemies were annihilated. That kind of action could easily cause a glitch, but even with fewer towers/mercs I was getting inconsistent damage. Without enemy info it's hard to understand why.
That all makes it ultimately a very non-strategic game. It's basically a random wave generator where even your careful expenditures are subject to randomization. So it sort of fails as tower defense.
BUT, I do like the money/gold/corn system, the allies setup (merc, wizard), and the theme. I think it could be good with a lot more work. Clear out the glitches and make the mechanics a little more apparent. I give it 2 for the premise and structure. If you can get rid of the glitches and just make it run more smoothly, it could easily be 4 or 5.