Base Defense

Instructions:

mouse or touch screen

Don't just spray your shots at whatever's closest. Pick off the fastest-moving threats first, because they're the ones that slip past your defenses while you're busy reloading on something slower.
Watch the edges of the screen, not the center. Most waves sneak in from the sides right when you're focused on a big target in the middle, and that's exactly how a base gets overrun.
Save any burst-fire or special click patterns for clustered enemies. Wasting them on a single lone unit is basically throwing away free damage.
Pace yourself early on. The first couple levels feel slow, sure, but that's your window to get a feel for the aim sensitivity before things get chaotic.
If a level starts going sideways, retreat your focus to the weakest point of your base rather than trying to save every inch at once.

Aim with the mouse (or your finger on touch screens) and click to fire at incoming enemies.
Keep an eye on your base's health bar; it drops fast once enemies get close enough to attack directly.
Clear each wave completely before the next one rolls in, otherwise things stack up quick.
Progress through the levels one by one, each one a little meaner than the last.

Straightforward point-and-shoot mechanics that anyone can pick up in seconds, no tutorial slog required.
A steady climb in difficulty across multiple levels that keeps things from ever feeling stale.
Sci-fi enemy designs that actually feel threatening instead of just reskinned dots.
Works equally well with mouse clicks or touch input, so it doesn't matter what device you're on.
Short, replayable rounds that make it easy to squeeze in "just one more level" before putting it down.