On arrival at the castle / convent, I discovered that entry is free on Sundays - bonus!
Then in the 14th century, the pope was getting a little upset with this group having such a strength, so he decided to dissolve them, and ordered that the Knights Templar be no more, and simply took all of their land and possessions.
Now, it seems that the then King of Portugal, while obviously being a good and devout Christian (and Roman Catholic at that), he was still a King, and there is only so much of Rome's orders which a King will take - for further reference on this idea, see England's King Henry VIII; so, as only a King will do, he came up with a cunning plan - he ordered that there be a new religious order, being the Order of Christ. Now, here's the good bit - it was all the same people who had been Knights Templar, in the same place, with basically the same rules, regulations and ideals, doing much the same thing as they had always been doing - for this too was a military-religious order. Nice work old King!
And so, the Knights' castle in Tomar was "handed over" to the Order of Christ, who promptly renamed it (under the King's orders) to the Convento de Cristo - or the Convent of Christ. The fascinating thing about the name is that normally a convent is for the nuns, while monks and the like have a monastery; nobody knows why it was named a convent and not a monastery, but my theory is that they were trying to get the military-religious guys in touch with their feminine side.
Over the years, and with much meddling by the various kings of the times, the convent was further added to and modified, to what we see today. It is a warren of buildings and cloisters, set around the central church, which is an interesting round shape, as constructed by the Knights Templar.