In a large bowl, mash chickpeas. Add the remaining ingredients and knead the mixture. If needed, add the reserved chickpea cooking water to the mixture.
Mold the mixture into long patties and set aside.
Heat the olive oil in a deep frying pan and fry the patties for 4-5 minutes on each side, until browned.
To make the yogurt sauce, simply place all ingredients in a bowl and mix well.
Spread some yogurt sauce on each pita. Add the sliced fennel and watermelon, 2-3 falafels – depending on their size, crumbled feta cheese and pistachios. Roll the pita into a wrap and serve.