Tough idea to work with.
I'm not trying to ruin your day with such a low score, but you have a lot to work on in this flash before it can actually be made into something great - which is exactly why I'm taking the time to write up this review for you. The way I review is by going on a scale of every category I find important in a flash. Don't be discouraged by the scores, use them as motivation for improvement.
Graphics/Animation: 2
The game really falls short here because there wasn't much pretty scenery to look at as a viewer. You need to make the enemies better looking, and have them doing a little more than the simple movements they're doing. The landscape isn't bad, but it needs more detail to sharpen the overall look of the battlefield up for a higher score. Another important thing would be to make the walls have your own type of design so that they don't look like Tetris blocks. Try to make everything as unique as possible so that this can reach a higher overall score. Everything needs to be touched up in my own opinion when it comes to the look of the actual game. Even the ending screen (and yes I understand this is a Beta version) looks like total garbage. You could have put a little more effort into the message that tells us that it's the end of the Beta. Improve in all aspects, and you'll have a hell of a game.
Audio: 0
There was no Audio, which is pretty bad. Even for a Beta version you need to impress your audience with some sound. You really need to include sound for the gun, the enemies, and even some music in the background. I can't emphasize enough the importance of sound in a flash. There's nothing else to really say here except for you to include sound, whether it's a Beta version or the full game.
Originality: 2
I couldn't give you a full originality score because this wasn't a full game. You need to improve in a lot of areas, which I will have to actually see the improvement in before I can rate your originality level. As of right now there isn't much about this flash that is unique and I can grade you on other than the idea of setting up walls. Use it, improve on it.
Gameplay: 1
The actual gameplay is the area where this falls the most short. For a game to be successful, you have to look at what most audiences like to see. Tower games are EXTREMELY popular, and can nail you a high score with no problem. However, this game was missing a lot of important things. You cannot make it so that the crosshair dead on the enemy is the only way to kill enemies, and then have a 30 enemy rush on a small battlefield. It completely kills the idea of the game in the first place. The idea to set up walls is extremely useful, but you need to do something like have a way to change the direction of the wall itself. There's an incredible amount of things you can do with this flash to make it better. Be unique, and improve in every aspect and you'll do great things with it.
Overall: 2/10
This is a low score, but at the same time it's a chance for you to change up everything and make a good game out of what I've mentioned here. I'm not saying all that I've mentioned is what will get you a god-like flash, but it's what I've had experience of seeing. You have the potential to make a somewhat good game here, and I really feel you should use it. I really like the idea of setting up walls and I think you can do great things. Good luck.