The tower outta nowheres
The quest starts automatically after you reach Urialla village (M17,1) and overhear the conversations between common folk. To learn more, walk to the campfire and talk to one of the citizens. He will tell you about a tower that suddenly appeared on one of the nearby hills. Their druid with five other men walked inside and never came back. You must check what happened.
Go to the hill. Walk up the tower's stairs and open the door. Inside you will see a portal through which you must walk. You will find it in a dark room so it is a good idea to use a torch here. In the room nearby you will see a Golem on level 29. He will attack you if you get close. After defeating him don't forget to search through the room, there are many containers here.
Walk to the magic barrier behind which a mage from Kovir is trapped. To release both him and yourself you must find a book with the right spell. After the conversation the barrier on the other side of the room will disappear and you will be able to walk deeper into the tower. You will reach a sunken room. Swim through the tunnel to a crossroad, then turn left. After a while you will reach a large chamber. There first you will have to defeat 3 Alghouls on level 21, then a Gargoyle on 26 and finally a Werewolf on level 25. After defeating all the enemies a... cow will walk out of portal. Simply ignore it. Don't forget about taking valuable alchemy ingredients from the bodies of defeated enemies. Now walk to mage's working place on small elevation and take the library key from a small chest on the table.
Go to the second part of the sunken tunnel. Dive, on the left side of the door you will find a lock that can be opened with the key you obtained. Swim further and enter the library. Find the books with the witcher sense. The one that you're looking for is lying on the other side of the library and has GOG.com shop's logo on the cover. After you take it, a poisonous gas will be sprayed across the room, so you must quickly go back to the water and swim to the trapped mage. Give him the book and he will remove the barrier.
Now go to the heart of the tower. When you enter the main chamber, the mage will start removing the security. While he is doing that, you should defeat an earth elemental on level 27. After that, talk to the mage. He will teleport the tower and you will land in the sea near the tower. Additionally, you will receive a runestone and Moonblade sword from him.
After you swim to the shore, the questgiver will talk to you and thank you for solving the problem. He will offer you 64 crowns. You can take it or refuse. After that, another villages will ask you whether you have found his brother in the tower. You can tell him that he died or lie that you haven't seen him.
The path of warrior
To start this quest you must talk with the women in the center of the village (M17,1). You will learn about a trial that all men must pass to become warriors. Go to Gunnar to receive more information. He will tell you that to prove your manhood you must walk through two paths, one leading to mountains, other through a sunken tunnel. Go to the mountain top first. You might meet few sirens on the way. When fighting them, it is more important to watch your feet than dodging their attacks. You can easily fall down and die. Second thing you must watch out for is properly jumping above the rifts. Walk through the whole path of warrior can be seen on the movie attached.
After a while you will reach a damaged tower. You don't need to walk inside. However, if you're at level that allows you to kill a wraith on level 20, then you should do it. After beating few obstacles and enemies you will reach a chest with full bear school witcher gear set.
To continue the road to the top, jump on the road behind the destroyed stairs. Follow the road, it's very lineal and it's not possible to get lost. After more than a dozen of jumps you will finally reach the top of the mountain. Don't forget to "take" additional skill point from the Place of Power. Take the wrap from the totem in the middle and Ashrune sword from the chest nearby. Now slide down the mountain.
You will be in front of an entrance to the cave. Go deep inside until you reach a hole filled with water. Jump, dive and swim to the other side of the tunnel. Try to swim straight to reduce the distance. The tunnel is very long and you might not have enough oxygen if you waste even a little of time. When you reach the end of the tunnel, dive again and use witcher senses to search for a chest at the bottom. It should glow in red. Take the proof that you have completed the trail from inside. Now you must get out of the cave. You must jump from one rock ledge to another. At the exit from the cave you will meet 3 bandits on 14 experience level. Go back to Gunnar. You will receive armor, diagram and few crafting materials from him.