You can spend many pleasant hours wandering around this huge complex, a mixture of a castle and a monastery. It was founded in the 12th century by the Knights Templar, and then extended and enlarged throughout the following centuries, even after the Order of the Knights Templar was officially dissolved (which didn't really happen in Portugal, anyway; they just changed their name into Order of Christ). Today it's a great mixture of Gothic, Baroque and Manueline elements, with 8 cloisters and the fantastic octagonal church, built after the example of the Church of the Holy Sepulchre in Jerusalem. Tomar itself is a pretty town with a direct train connection to Lisbon. The walk from the station to the convent takes about 20 minutes, and only the very last part is uphill. It's a great site that you should take some time to explore.