pygame: space invaders

Collision detection and input

Simplified Space Invaders
https://www.mysteinbach.ca/game-zone/1755/alien-invaders/

You can simplify the invaders to have different shapes.
You can simplify the shape of the buildings.
The behavior of the game has to be the same:
a. The invaders move right to left as they move down.
b. The bullets destroy the invaders ( they disappear).
c. The bullets destroy the buildings ( they disappear).
d. Keep score and lives.
e. Final message: Game Over

Optional Challenge: Have different levels of difficulty

Simplified Space Invaders
space invaders