I think the idea and the way it's made is neat. I mean, underground army, that's already unique in the concept, and the gameplay itself is cool. Unfortunately for me, I found it difficult in the beginning. The interface confused me for a while, and I ended up spending some time figuring out how everything worked, and BAM "Wave 1" appears at the top.
Well, I'm going to play it again to check it out and see if I can overcome the oddity of this game. The soldiers travel pretty fast (rather odd, I thought), and if you get a bunch of soldiers and shoot a bunch of slow bullets (the soldiers are faster than the bullets...?), then you can retreat and the enemy runs into the bullets. Hm. Anyhow, this is a well-made action-strategy game. You might want to put more thought into the help section, or maybe make a tutorial. By the way, in the options menu, you misspelled "medium."
The most annoying thing is the sound. When enemies attack the base, the alarms are interesting at first, then it gets very, very irritating. But otherwise, the music is alright. Vaguely repetitive though.
Keep making good games like this.