I started playing and didn't think I was going to play for so long. I really had fun playing this. Sometimes my zombies would get stuck because there was a zombie in between them so they couldn't merge. I just had to select that one and move it but, found out I could make them all merge together. They would then separate on their own leaving a bunch of 100 level/HP zombies behind. 

I had a huge horde and attacked a castle and fort at the same time and froze my game. I should take it slow next time. I don't know what I would suggest to improve and I didn't see anything to call out as an issue. This hit a spot with me and I like it.

Overall, very playable and good! 

The only feature I'd like to see added is the mouse scroll instead of the center of the screen to use the location of the mouse, so that similar to other documents and google maps and such could scroll out in one place and zoom into another place instead of the panning function. 

I like the upgrades, they give some good depth of strategy. Once or twice I got the same option for two of them though.

A couple of things I didn't notice for a while, and very much influenced gameplay:

1. The towns gain strength. This is good cause the player has to come up with the most efficient way to gain strength faster than the towns.

2. How exactly the combat is calculated. I know this is an important part of your design, but you may even be able to get away with something dirt simple: The bigger number wins on the first encounter, and is left with the difference of the numbers.

3. That you could change the time scale. The default time scale is very slow, and so as a new player I almost bounced off of your game exclusively due to the crawling speed of movement. 

Ok, so the tactics I wound up favoring was just to quickly defeat all the towns together as one big horde, stacked as closely together as possible with at least two groups for flanking. I would prioritize the closest and easiest to get to towns.  

Is this the best strategy? Could you add circumstances where it is not? Otherwise it may get old soon.  

Your game is super fun and plays just right. You did a fine job on this!