Hilary, to fully answer your question I would have to go back to Richard the Lionheart and the Third Crusade, but that's a long story. At the risk of incurring the wrath of the Moderators by embarking on a History lesson on the Plant Forum.
Suffice it to say that calling the present building a castle is generous in the extreme, it's a fortified keep. It was built in 1454 and stands on the site of an earlier castle, possibly erected around 1210. In that year the Lusignon King of Cyprus, Hugh I, had given a vast tract of land, stretching from Paphos to Limassol and up into the foothills of the Troodos Range, to the Order of St John of Jerusalem (Knights Hospitallers). Following the fall of Acre in 1291 they were forced to move to Cyprus and in 1302 established their HQ in Kolossi. It officially became the site of their military administration, called Commandary, in 1380. What happened to the original castle is uncertain but the present one was erected in 1454 by the then Grand Commander, Louis de Magnac. The Grand Master of the Order had moved to the island of Rhodes in 1310, where his magnificent palace can be seen today.
As regards the sugar, it is believed that sugar cane was brought to the Middle East from Asia by Arab traders in the 10thC. It came to the attention of northern Europe via the Crusaders. Until then honey had been the main source of sweetener in that part of the world. When the Hospitallers moved to Cyprus they brought sugar cane with them and grew it in large areas of their vast land holdings. The sugar mill and associated factory date from the 14thC. The mill was powered by water brought from the nearby Kouris river. Three types of sugar were produced and depended on how many times the product was boiled; pulvere di zucchero - pure, refined sugar powder; zamburo - less refined and molassa - a syrupy mass. However, due to a variety of factors sugar production declined in the 16thC.