Grammar: Fill in: by and with.

Solution:

1. The building was designed by an architect.
2. The walls were painted with red paint.
3. The monument is visited by many tourists every day.
4. The pyramids were built by the Egyptians.
5. The statues were covered with mud.

Explanation: We use 'by' to indicate the agent performing the action in a passive sentence and 'with' to indicate the instrument or material used.