Defending Kaer Morhen

Defending Kaer Morhen is the main quest of the Prologue of The Witcher, set in the witchers’ fortress of Kaer Morhen. Bandits storm the keep, and Geralt, freshly returned and missing his memory, fights alongside Vesemir, Leo, Lambert, Eskel and Triss to drive them out, with a frightener and a mage thrown in for good measure.

The quest is also the game’s combat tutorial: it introduces the strong, fast and group styles, potions and the Aard sign as Geralt pushes through the courtyards, opens the upper gate from the tower and fights his way into the keep and down to the laboratory. Midway through, he has to decide whether to help Vesemir against the frightener or Triss in the lab. Either way the raid ends badly: the attackers escape through a portal with the witchers’ mutagens, Triss is wounded, and Leo is killed by the Professor.

Walkthrough

Pick up the rusty sword and fight the first bandits outside the castle using the strong style.
Follow the witchers to the lower gate, then circle round to the north to reach the winch of the upper gate, learning the fast style on the way (strong for big hooded bandits, fast for small ones and archers).
Climb the tower and work the winch. Triss drops the gate on the frightener, and Vesemir gives you Thunderbolt and Swallow potions; drink Thunderbolt before entering the keep.
Fight through the keep with Leo and learn the group style.
In the laboratory, find Triss to break the mage’s barrier. With the stairs blocked, follow Leo to the Circle of Elements, learn Aard and blast the rubble.
Choose whether to help Vesemir with the frightener or Triss in the lab (see consequences).
After Leo’s death, talk to Vesemir and meditate to spend your first skill points; this leads into A Potion for Triss.

Quest Consequences & Choices

Help Vesemir: you face the frightener in the courtyard. Cast Aard at the siege cauldrons and the bell; it can only be hurt while all three ring.
Help Triss: you fight the mage Savolla in the keep. Kill the bandits feeding his shield first, then knock him down with Aard or use the strong style.
Both paths give a Tawny Owl potion and end the same way. Once the Prologue is over you can never return to Kaer Morhen, so loot it thoroughly.