Ambrosia is a lost continent, as large as Sosaria and filled with much of the same terrain. You will not find any towns or dungeons within Ambrosia, only four shrines hidden deep within pockets of the realm. There is only one means of reaching Ambrosia, and that is by sailing in to the heart of a whirlpool. It will suck you down into this mysterious realm beneath the sea, where you must fend for yourselves until you escape. Returning to Sosaria can be equally challenging.
If you do not wish to learn any more about Ambrosia before you have a chance to explore it for yourself, do not read the sections below, since they reveal information about important locations found throughout Ambrosia.
Access and Escape
In order to access the land of Ambrosia, you must acquire a ship in Sosaria, and locate a whirlpool. The whirlpool moves about the surface of the ocean, and can be difficult to track down on occasion. When you locate it, sail straight into it. You will arrive near the center of the southern portion of Ambrosia, just above a lake of water.
In order to leave Ambrosia and return to Sosaria, you must encounter a pirate ship along the southern lake, attack its crew, and gain control of the ship. Then locate the whirlpool found in the lake. You will be deposited right outside of the Castle of Lord British.
Be aware that, although your curiosity may be piqued by the presence of several doors found throughout Ambrosia, there is no benefit to opening them and searching for whatever lies beyond them (except for one door in the NES version as described below).
NES version changes
However, the NES version of the game provides an alternate means of returning to Sosaria which drastically cuts down on the effort you must make. Just west of the heart of Ambrosia, you can locate a tile which will provide you with a flower. If you collect this flower, and provide it to a woman named Sherry in the Royal City (the Town of Britain), she will reward you with a Heart Compass. When activated, the Heart Compass will instantly return you to Lord British's Castle without the need to board a ship and sail into a whirlpool.
A single Flower can be carried by each character, and each character is capable of carrying a single Heart Compass in return. The Compass works anywhere in Sosaria, Ambrosia, or even in a Dungeon (have not confirmed if it works in Exodus). The Flower and Compass can effectively give you 4 teleports home, and is easily replenished with each trip to Ambrosia.
One more feature unique to the NES version is the existence of an Inn in the northern holds of Ambrosia. There you can save your progress without fear of losing the accomplishment of ever reaching Ambrosia in the first place, though reaching the Inn can be quite a feat on its own, and will require a key to open the door that it is hidden behind.
Shrines and Cards
In Ambrosia, you can locate four shrines, each of which are associated with a particular attribute. At the cost of 100 Gold, you can raise one of your characters attributes by one point, up to the maximum amount allowed by his or her race. Naturally, it is a good idea to bring as much money as possible with you in order to limit the number of trips you must make to this land.
Increasing Strength and Dexterity will transform your party members in to far more capable fighters, while Wisdom and Intelligence need only be raised by those who cast spells associated with those particular attributes. In other words, a character which does not cast Wizard spells gains no benefit by raising his or her Intelligence, nor will raising Wisdom benefit any character not capable of casting Cleric spells.
Lastly, the Shrines hold one other item of significance: the cards. Collection of all four cards is essential for the completion of the game. Each shrine contains a different card. You must search the Shrines in order to locate cards. In the Shrine of Strength, you will find the Love card, in the Shrine of Intelligence, the Moon card. The Death card is found in the Shrine of Wisdom, and the Shrine of Dexterity has the Sun (or Sol) card.