Tough game..?

[Pooter90]

Ok, I've been playing for a little while now and my top score is just over 1000... I don't get it. Do I just suck terribly? Or is everyone here talking about getting high scores simply amazing?

Also, sometimes I'm unable to sell towers, why is this?

Edit - Omg, 1285... Man, I thought I was doing really well but I just couldn't keep up, despite all the layers of towers I had. It seems like once I get past 6-700 or so, I use up all my money buying towers again since those guys go through them like a hot knife through butter. By the time I get around 1000, it's impossible to keep up, and I have fewer and fewer towers each wave.