There are a few different types of doors that Wonder Boy will encounter in his travels. Here are their definitions.
Door with Window: Contains Shops where Wonder Boy may buy equipment.
Regular Door: Some rooms with treasure chests or hospitals have this kind of door. Some doors lead to unexpected destinations.
Door with green lock: Cannot be opened without a key. Once opened, the key is no longer needed.
Door with red lock: Cannot be opened without a key. Even when opened, cannot be reopened without a key.
Certain blocks give Wonder Boy specific abilities and may contain or lead him to treasures.
Jump Block: A block with an arrow. When you jump on top of this block, you are sprung high into the air.
Mouse Block: Checker-patterned block. The Mouse-Man can walk on the walls and ceilings made of these blocks.
Destructible Block: Can be destroyed with the Thunder Saber.