Worlds of Ultima: The Savage Empire/The underground Kotl ruins

The outer gate
The device to open the first Kotl gate is at the northmost end of the Great Mesa. [U]se Aiela's giant gem on it to activate it.

The first Kotl gate is north-east from the device, just slightly to the south-east of the Kotl teleporting hub. Anyway, you have to backtrack south to get off the mesas, then travel north again.

The inner gate
[U]se the metal head on the golden man, then talk to him and ask to swing the door open. If you have the crystal brain in the inventory, he will tell you even more.

Yunapotli is a peculiar companion. He needs no armor, because he is invunlerable in Kotl; you can set his combat tactic to "close", so he will tend to be a wall between the companions and the most dangerous enemies; he needs no weapon , because he is already at level 8, i.e. he doesn't need to gain experience points; his very low dexterity makes him a poor fighter, because he is so slow that he will seldom get a turn in battle; but his high strength allows him to carry lots of items.

The Kotl ruins
The Kotl city was organized in four districts around a central square. The entrance is in the middle of said square, where the Avatar can meet Katal-kotl.

Item list

 * Equipment: Kotl shields, bombs and black staves are all essential to beat the Myrmidex.
 * Food: there's no day-night cycle in the city, so rest and eat food whenever your characters need to restore some health points.
 * Useful items: the Device is a vital "compass" to find your way in the final dungeon; healing crystals are useful but, alas, they are verye heavy: 4 kg each; even the strongest companion can carry just a few.
 * Other items: move all the pillows, because many useful items are hidden under them.

Character dialogue trees
If you missed the wisp in the north district, you can meet it again in the Urali swamp, north of the Mighty Lizard isle.