Principal and Principle

The adjective principal means the most important, first, or main.

The old castle is the principal attraction in the city.

The castle is the city's main attraction.

The noun principle means a basic truth or theory; or a belief or rule which guides behaviour.

It was against her principles to lie.

She didn't lie because it was against her beliefs.

The investment plan was based on economic principles.

The plan was based on economic theories.