While the internet is abundantly saturated with Flash based Defend-the-Castle games, none of them are good.
Base Invaders marks the first time I've been impressed with a defense game and the main reason why it succeeds where others have failed is because of its unique traps, great story-telling cutscenes, and its unbelievable graphic quality.
Base Invaders plays out much like an RTS game. You've got your basic RTS-style movement with the mouse. You can scroll around the map, place traps, and fling enemies. Killing enemies earns you money, which you then use to buy traps.
The traps are the epitome of cleverness. Traps range from the straight-forward hammers, fans, and springs to downright nifty ones like flamethrowers, vortexes, flash-bulbs, and Tesla Coils (there are much more).
The enemies also come in a large variety, each with their own unique aspect. Your basic grunt is nothing special, just a cute green tubby character that moves slowly and possesses weak attacks. But if you think that's it, you're crazy. There are other enemy types such as the ninja, which jumps over walls, kamikaze bombers, which explode on contact with anything, miners, which break down walls with ease, and the incredibly annoying Spikers, which can't be picked up manually and must be destroyed with traps.
Each mission is filled with various cutscenes introducing new traps and enemies. The dialogue is well written and the little comments from the invaders are quite humorous. The graphics are also top notch with very colorful textures and good particle effects.
The game can be pretty hard and there's a fairly steep difficulty curve, so those who aren't familiar with fast paced mouse clicking should beware.
Overall, I highly recommend this game to fans of twitch-based action, however, I don't feel this is worth more than a coffee break to those who aren't.