It's hard to find the little white castle with the round tower and black witch's-hat roof unless you're looking for it. It's hidden under some trees on Memorial Drive near Beltway 8; nevertheless, discerning diners from all over the country are turning up here. That's because Indika has already earned a reputation as one of the most innovative Indian restaurants in the United States. Outstanding entrées include duck tandoori in a toasted almond curry served with haricots verts and fluffy white basmati rice, and half a roasted eggplant filled with paneer and cashews. But first try the crab samosas with papaya-ginger chutney, and don't forget to save room for the best nan bread you have ever eaten, served with a spicy side dish of yellow lentils flavored with garlic, ginger and cumin.