One of my favorite games! I used to play this way back in 4th grade.
For 2004, this is about as good as Flash games get. I really like the animation here - simple and smooth. You've got some really nice effects too; in particular the exhaust from the spaceships as they crash are very impressive. The sound effects and music are good although they do get old after a while - very common for games of this era, however.
The gameplay is really wonderful. Slinging around stick figures and tanks is always fun, although I've noticed that unlike the original Defend Your Castle, your stick figures fall back to the ground immediately after you let up on the mouse - instead of 'throwing' them you're actually 'carrying' them. The spells are a really nice addition, and I like that the difficulty can be altered at will as well.
It would be nice if there were more things to unlock or more incentives as you played through the game - perhaps bosses (as I know there are in DD4) or more enemies that came along later. As it was, I made it to level 65 easily and by that point was only stalling before inevitably using either Nuke or the rest of the spells on the same page. Repeatedly spamming spells and restocking runes gets repetitive when you reach that part of the game - especially if you've bought everything, have 30,000 HP, 500 of each rune, and it costs 50,000 just to upgrade cursor strength. Long story short this gets quite repetitive and isn't built to last for more than a few dozen levels before the formula kind of breaks the game. However, I still really like this and the game remains fun to go back and play even after nearly a decade.