The Venus Lighthouse is located south-west of Lalivero. The Venus Lighthouse's enterance is actually only accesible from underground. The Venus Lighthouse that appears aboveground is actually the exit. Golden Sun: The Lost Age begins at the Venus Lighthouse while Golden Sun ends at Venus Lighthouse. That being the case, it is fairly obvious that Venus Lighthouse is the final dungeon in the first Golden Sun.
Venus Lighthouse is also the location of the final boss in Golden Sun; Fusion Dragon which is Saturos and Menardi. This is the final place where you are able to see Saturos and Menardi before they transform into the dragon. Venus Lighthouse has a good abundance of high leveled monsters, and has a few weapons that you should pick up before you fight the final boss. Make sure you have the Gaia Blade for Isaac and the other weapons and armor for each other character. Try to fell a Ferinir to get a very powerful blade, and open all the treasure chests you can. This is the final fight, so after this you will probably transfer the data to the Golden Sun: The Lost Age so make sure you have things like all the Djinn and the Cleric's Ring.
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