Castle Town - Piece of Heart
Surprisingly enough there is only a single Piece of Heart for you to acquire within Castle Town. Many Heart Pieces have you travel throughout, of course - but only a single one actually comes from Castle Town. That Heart Piece is given to us by the Old Man near the same entrance that has the Castle Town portal.
This old man asks you to make a donation for peace in Hyrule... Yea, cause you can buy world peace right? You can only donate 30 or 50 Rupees, choose whichever you'd like but keep in mind you have to donate a total of 1000 Rupees in order to get this piece of heart.
If you need help getting all the Rupees together that you need to purchase this piece of heart I recommend you collect the different Golden Bugs and bring them to Agitha. Not only will she upgrade your wallet size but she'll solve your lack of Rupees problem extremely quick!