Do a search for desktop tower defense. You will find the game this is based on. That might help.
Unplayable 
I put some tower…
I press space, the level turn to 1…
And nothing… it seems that it take several minutes before something appends ???
Windows XP sp3, Celeron 2.40 GHz, 512 RAM, Java 1.6, Intel 82865G
Try on a P4 3,20Ghz, Windows XP SP2, Java 1.6 and GeForce 7800GS. It works fine.
Great game 
This works on my home computer.
Java 1.6.0_11
Windows XP
AMD64 3000+
GeForce 6800
My only complaint is that you need to make the field half a tower size taller and half a tower size wider. This will let me make a better maze.
Otherwise good job, it’s pretty fun.
Regarding the “frozen” behavious. I had a similar issue with Grasshopper4k and the solution seems to be to add a Thread.sleep([sometime]) to give the AWT Thread some room to make its work.