Its alright but it needs work
Some ideas for improvement...
1)ability to unselect units
2)Repairable;e units
3)Mute option
4)alternative maps
5)building upgrades that branch out into other categories
some not as necessary ideas are...
1)a build map ability
2)variety of looks for you enemies
3)support buildings like
-buildings that enhance other building
-buildings or upgrades that increase income
4)building with special attacks (ie critical strike for a sniper post)
5)traps or obstacles (ie barbed wire or tank traps)
6) multiple enemy routes and/or splitting routes
I noticed the concept it no quite original but i like that you chose the a more realistic theme instead of ice towers and stuff like that. try some of this improvement as well as others peoples' good luck on achieving your goal.