It seems like I'm on a kick with Space Invader type games (recently reviewed Battlantis). Hole Land is a short, sweet SI imitator with fun gameplay made drastically harder by the holes (thus the name - "Hole Land").
You're a space dude shooting up at the approaching (and encroaching) invaders, but as they move across and down the playfield, they constantly jump in and out of holes. You can only kill them when they are completely exposed. It makes for a lot of missed shots and low-level frustration. You actually have two buttons to work with, not just "fire". You can also jump. The reason you jump is to avoid and jump over the enemy aliens once they reach the bottom and run across it to kill you. As they run sideways toward you, simply jump over them or land on top of them to kill them.
In fact, being run into by an alien is the only way to die (it cuts you in half). However, there is one other major impediment to your success. The enemies fire and toss things at you constantly. Luckily though, these things don't kill you, they simply deflate you and you become immobile. Then a repair dude comes out and pumps you back up so you can continue to fight (I'm not making this crap up). Actually, you can even help the process along... by pushing the fire button as the repair man re-inflates you, you speed up the process. Once you are re-inflated, you can move and fire again, but the problem is the aliens have continued to advance while you were incapacitated.
Another weird quirk to the gameplay is that when you die on a level, you are then reincarnated on the next level - skipping the rest of the level you were on.
Hole Land is extremely short on levels. After you finish level 3 - which is different in flow and play then the other two levels (more of a swirly, abstract alien attack) - a big, ugly boss comes out and you fight him. Defeat him and it's back to stage one.
Level 3 is so special to the game that the dip switches have a difficulty level setting JUST for level 3.