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

The Kotl city gates
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 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 Yunapotli and Katal-kotl. The Kotl lizardmen district is to the north; an elusive Xorinite wisp can be met there. the Myrmidex district, now closed down, is to the west. The generators district is to the east; Spector blocks the access. The labyrintine human district is to the south.

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. Still, make sure you load Yunapotli with the less useful items (no spoilers here).

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.