@Superku: I feel the same about some points. Here are my plans for the next versions:

Its often annoying that people want to build for so long. I'm going to try out Superkus idea of fixed timers, too see how people react on that. I'd rather have a longer build time, maybe ranging from 30 seconds in the first rounds to 180 seconds in the higher rounds. I'll add sprinting until then so that you can build much faster, and the monsters will take less hits so you loose less crates and can continuously increase your fortress during the rounds.

Still I'll add more build variety, the generator will have a key part:

You'll need to power it by putting an oil crate on top of it. It will then create electricity which can transmitted though metal crates and will power turrets and other gadgets, possibly. The oil crate has to be replaced every now and then because it runs empty. You'll also be able to create traps by using bait-crates which will draw all nearby enemies to them for a limited time. You'll then be able to concentrate your fire or use TNT to get rid of them.
Later, you'll be able to create fire walls by dropping oil on the floor and igniting it, but that won't added to the next version.

I'll see what I can do about the weapons, they'll surely get some shoot trails/tracers soon. In the long run, I'm planning a weapon upgrade system so people will be able to specify themselves for a gun. Together with head-shots this will make the pistols and smaller rifle more attractive because of their accuracy. Limited ammunition is also something I'm aiming for, and I do like Superkus idea, because it doesn't involve buying stuff, which I don't really want to include.

Boss waves are also planned, but might not make it into the next update.

As I said before, I hardly have any time right now until the end of February. The next version will be released in march, hopefully featuring most of the above!