According to the Bible, the Tower of Siloam was an ancient tower in Siloam in south Jerusalem. In Luke 13:1-5, in a discourse on the need for individual repentance for sin, Jesus refers to 18 who died when the Tower of Siloam fell on them. Apparently they were innocent victims of a calamity which was due to no fault of those killed.
The Tower of Siloam itself is mentioned only in Luke 13:4 as part of a larger passage contained in verses 13:1-5.
"Some who were present" reported to Jesus that the cruel ruler Pontius Pilate had killed some Galileans while they were worshiping. Their example was particularly gruesome since at the moment the Galileans were killed, they were worshiping God by offering sacrifices according to their Jewish religious law.
Apparently those making the report were looking for Jesus to offer some explanation of why bad things happen to good people—in this case even while they were worshiping. The "sin and calamity" issue involves a presumption that an extraordinary tragedy in some way must signify extraordinary guilt. It assumes that a victim must have done something terrible for God to allow something so tragic to happen to them.
Jesus responded as though the question was related to the relative sinfulness of the victims, but only in passing. Instead, he diverted the focus onto the interrogators, making it clear that the most urgent issue was not an explanation to their implied question as to why God allowed such a horrible thing to happen to innocent people. Rather, Jesus wanted them to focus on their own souls.
Do you think that these Galileans were worse sinners than all the other Galileans, because they suffered in this way? No, I tell you; but unless you repent, you will all likewise perish. Or those eighteen on whom the tower in Siloam fell and killed them: do you think that they were worse offenders than all the others who lived in Jerusalem? No, I tell you; but unless you repent, you will all likewise perish.”
His mention of the fall of the Tower of Siloam added a nuance to his prior point: accidents happen. Therefore, even in the absence of persecution, death can come unexpectedly to anyone, irrespective of how righteous or how sinful they are. He may have been emphasizing that the time granted by God for repentance is limited.