In Spain, you'll find that most ATMs ("cajero automático" in Spanish, "caixer" in Catalan) have a withdrawing fee for international cards like Visa/Mastercard. ATMs are legally required to indicate the exact fee and ask for confirmation. So feel free to test around, cancel the transaction if the fee is too high for you and take some time to find the best ATM to use.
Read the comments for many updated ATM fees, and let us know in the comments if you found a no fees ATM or if you had different fees with your card.
Your own bank may also add its own fees. This is specific for each bank, so check with your bank pricing to see the fees for international withdrawals.
These ATMs have popped up everywhere in the center of touristic cities in Europe: do not use them. They have horrible fees and push users to withdraw high amounts of cash. Moreover the dynamic exchange rate (if you choose to let the ATM make the conversion for you, never do that) is outrageous.
Don't forget to ask for a receipt, especially when you're abroad. And once you have it, take a picture of it immediately because you'll probably end up losing the paper. Make sure the amount on the receipt matches the amount on your account. And in the case of a dispute, use your receipt/picture as a proof.
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Walk around the medieval Old Town and travel in time with the stories and legends from the past. See the most important monuments of the Gothic quarter. At the end of your trip, relax with a glass of Sangria in the Boqueria Market.