The following tricks work on the Famicom/NES conversion and most of its variants including the Famicom Mini version published in Japan for the GBA, and the Wii Virtual Console version.
Continue
At the title screen, press and hold 
and 
three times, and press 
Stage Select
At the title screen, press and hold and enter . Then release and enter 
. (Note that you only need to hold right, not the other directions.) Then press
You will be presented with a stage select screen. Use 
to advance the levels and to rewind the levels. Press to begin on the selected level. When you finish the level, you will be asked which level you wish to be taken to next. Note that you can choose Stage 7, and you will begin with the Lance. You will be able to defeat Astaroth with the Lance, which makes this fight easier. Also, you can select Stage 7 two times back to back to witness the ending of the game very easily.
Secret Message
In order to see this message, you must defeat Astaroth twice and see the real ending of the game. At the second screen of the ending, enter . If done right, you will see the screen presented on the right.