There are a lot of spiders on this level...Giant Intelligent Friendly Talking Spiders... I've always had sympathy for the people that were driven insane by these spiders.
Head over to the west right away and you'll see a cracked section of wall. Instead of using Move Mountains, just go around the corner and through the door. There's an empty chest in this room, and right next to the chest is a secret passage. At the end of the secret passage are some chests, a couple of which contain some halfway decent items.
Now leave this room and go around the walls of the cave in a clockwise direction. The first passage to the west leads to a pool with some Healing Herbs on the ground.
Ignore the room with the stairs leading up to the door for now and head over to the northwest corner. This is the spider's larder. Don't free the roach or kill it, as it might upset the spiders.
Continue heading in a clockwise direction and the next passage leading off to the north contains first a Guard Spider, who you can talk your way past without any problems, and then, in the room at the end, you'll find the Chief Spider. He'll tell you about the Friendly Roaches, but before he tells you where you can find them he wants you to do him a favor. Once you've helped out the spiders near the rocks come back and he'll tell you where to find the Friendly Roaches.
Continuing to head clockwise you'll notice some coins lying on the ground, which you might as well pick up.
The next passage to the north is blocked by some webs, and leads to a bunch of roaches that are being held for snacking purposes. You can kill them without upsetting the spiders.
The next passage off the main room, which leads to the west is just a small side room with two spiders in it. Feel free to talk to them, if you're so inclined.
The last passage leading off the main chamber is blocked by a pile of rubble, which you need Move Mountains Lv.3 to get past. Fortunately for you, there's a secret passage. Go over to near the entrance and start checking the southern wall. This secret passage takes you into a room with a lot of bars of iron and some spirits. Once you've dealt with the spirits head over to the northeastern corner of the room to find an Item Augmentation Platform.
Now go check out that room that has the stairs leading up to the door. When you first enter a Demon and four Imps will appear. The only things in here are a plain Gold Ring and an Iron Dagger, but you can come back once you've gotten the Ritual of Sanctification ability and use it on this altar. Performing the Ritual here will cause the appearance of a Haakai who will drop the Obsidian Spear when you kill him.
Now that you're done here, it's time to head up to the home of the Giant Intelligent Friendly Talking Roaches. They're even more creepy than the spiders.