Fill a good-sized pot with the water and set it on a high heat. Add the chilies, lemongrass, garlic, galangal, lime leaves, palm sugar, lime juice, and fish sauce. Bring the pot to the boil and then immediately turn the heat to low so that it runs at a gentle simmer for fifteen minutes. Lemongrass is a pretty woody, fibrous plant and this simmering is needed to bring out its scented flavor. It will also soften the lime leaves.