SHOOTIFENCE

While it looks like a shooting game, it's essentially a tower defense game.
You continuously defeat various types of enemies that appear from above without missing a single shot, while infinitely upgrading three types of weapons with different characteristics.
The three weapon types are: the Vulcan, which has rapid fire and high ammo capacity; the Laser, which has penetrating power but no rapid fire; and the Missile, which explodes on impact and deals damage to surrounding enemies.
Each can be equipped separately on the left and right sides of your ship and upgraded independently.
There is compatibility between enemies and weapons, so depending on which enemies appear, your currently equipped weapons may be more or less effective against them compared to other enemies.
The enemy placements, weapon change options that appear, weapon upgrade options, and other elements are fixed on a monthly basis. The patterns change when the month changes.

Ultimately, it's likely that the weapon change options themselves, rather than the enemies, will become your greatest adversary...