Thanks NichoMath, to play the gave and give us some feedback, it's nice to see that you enjoy it, and also thanks for referring us!
About the soldier progression we have it in our TODO list, and also we want to add new types of soldiers, but first we are focusing on the zombies side, we will add it, but give us some time and stay tuned!
About the movement of the hordes, we are currently working in a new spawn system and some AI improvements, so you will be able to check it in future versions :)
And yes! We will keep working as hard as possible to develop a great game!