Another common question - and one where opinions differ. Just use the 'Search' facility (above right) and type in "exchange" and you will find lots of topics in our forum.
Personally, I try to avoid using cash at all costs - although you need to be aware that payment by cards is not as widespread as you will expect at home and if you intend using a card, always ensure that you have some form of identification (photo driver licence or passport) with you to prove you are the card owner. Anyway, my advice would be to get US$100 converted to TTD before you leave, then use a MasterCard/VISA card to draw cash from ATMs on Tobago, as and when you need it. In fairness, this arrangement is only financially beneficial when your card company doesn't make high charges when it is used for drawing cash and/or if you have a zero balance and can pay off before the due date so that you don't pay the exhorbitant interest rates on the cash.
There is no bank in Castara - but there is an ATM machine.
Most properties have a security safe suitable for cash. So, best to check with your accommodation provider.
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