The Eldar stronghold definitely shows the Eldar's love of misdirection and deception.
At mission start, you do not have a base but you do have a moderate army alongside your hero and honor guard, and you need to destroy a small Eldar base to the south. Once you do this you will also gain builder units and can start building your main base in the center of the map. A second enemy base will be revealed to you which you need to destroy. Once you do, yet a THIRD base will be revealed in the northwest corner of the map, but you will not be able to access it by normal means. You will need to find and destroy a series of generators that are projecting false rock walls blocking your access to the final base; the paths to these generators are also protected by stronger-than-normal defense turrets. The defense turrets can be destroyed by force (not recommended) or disabled by destroying a Thermo Plasma Generator hidden in the northeast. The final base is also completely infiltrated.
Some special tactics that are possible on this mission:
-If you are using a race that has both good anti-all air units and enough ground forces with good jetpacks/teleporters and detection to get inside the base you could attack it directly, and bypass the need to destroy the generators. The Tau are capable of this if the commander has the detection and jump pack wargear (and preferably at least cloak so he'll live long enough to do something) accompanied by a number of Scouts upgraded to have good anti-building weapons, and the Imperial Guard have the option of building three artillery batteries and pounding the area with Earthshaker rounds.
-The Necrons are the kings of beating the final base; if the Necron Lord has the detector wargear his teleport has enough range to get inside the base. Use the Monolith Stomp technique with a full infantry army of Warriors and Immortals, wait the 1 minute for the Lord's transformation to wear off (Detection is lost during the transformation to the Deceiver), and then let rip :).