1.) Is the process of building and upgrading a tower intuitive? Do you have a good sense of the range of a tower?
The building and range was intuitive and I liked it. However I was unsure what upgrading did. Was is just health or was it range and damage as well.
2.) Besides a bit more information regarding the specifics of towers and upgrades, is there anything you feel would improve the user interface?
A pile of rubble would be better than the red tiles in my opinion as it would nicely wrap up the graphical side of the program.
3.) How do you feel about the difficulty of the game? We aimed for the prototype to have a rather high difficulty, as we wanted players to hit an end point (losing). Compared to this, what sort of difficulty would you like to see in future versions (Easier, much easier, about the same, harder because I'm a masochist, etc.)?
At this time it is a fair game with me reaching wave 20. I would like to see the game gain in difficulty over time and maybe additional towers unlocked over time.
4.) It's hard to tell (especially with some graphical glitches we're still running into), but each tower does subtly show its health. Did you notice this fact, and if you did was the display useful?
Once I knew where the health indicators were it was easy. I rather liked this method as it makes you double check the status of the towers.