I highly recommend taking a guided tour! Retiro Park is very large, and the various attractions are somewhat spread out. Unless you're planning to save money (the park itself is free) and are willing to spend more than half a day slowly strolling and exploring, having a professional guide will be more efficient and enjoyable. Our guide was very dedicated, explaining Spain's history to us and guiding us through all the park's attractions (he even went out of his way to take us to additional places we wanted to visit). It was a wonderful experience!