Twilight Princess tips and tricks
The Hot Spring Water Quest
Here is a small quest that stirs up a lot of questions, I'm gathering here all the data that is in the walkthrough and in the tips and tricks.
First step : Finance the repair of the bridge
After completing Lakebed Temple (and after talking to the Goron in front of the bridge west of Castle Town???), go back to Kakariko and enter Malo Mart. Talk to the old Goron inside who's asking for donations and give him 1000 rupees, in several payments, to have the western bridge of Castle Town repaired.
Second step : Open a Hot Spring Water booth in Castle Town
Talk to the strange Goron in front of Kakariko's Malo Mart and accept to transport hot spring water. You then have to bring a barrel to the Goron in front of the western bridge of Castle Town. Cross the field while avoiding/fighting the enemies. The best way to avoid the spawning of enemies (particularly of Leevers that spawn in groups and that are quite annoying) is to walk along the east part of the field, close to the cliff. Obviously, the barrel has to stay whole! Once you are close to the Goron, stand behind him and throw the barrel on his back. As a thank-you present the Goron offers you a Piece of Heart.
The booth is then open in Castle Town's South Road.
Note: If you finished this step before giving the 2000 rupees to the old Goron for the construction of the Malo Mart branch, he will only ask you for 200 rupees!
Third step : Bring water to the Goron in the southern part of Castle Town
Now that the booth is open in South Road, you can buy Hot Spring Water for 20 rupees. Fill a bottle with Hot Spring Water, leave town by the southern gate, go down the stairs and talk to the Goron at the far south. He will start digging a pathway in the boulders. Come back the day after to see a pathway towards Hyrule Field.
Note: It's no use trying with hot spring water from the Kakariko spring, even by warping, there is not enough time to come and go before the water loses its benefits.
Opening of the castle branch of Malo Mart
In order for this shop to open (and to replace the luxury shop) on Castle Town Central Square, you have to make more donations to the old Goron being in Malo Mart in Kakariko. If you have already performed step 2, he will ask 200 rupees, otherwise you will have to pay 2000 rupees!
Thanks to Baffy for his clarifications.