Towers Block is any set with Towers Sites (4, 5 and 6 only). Towers Standard is any set that was released by that time (so sets 1 to 6, but with Towers Block sites).
Similarly, King Block is any set with King Block sites (7, 8 and 10, not 9), whereas King Standard is any set released at the time RotK (set 7) came out (so sets 1-7, though King Block sites only). It's more of a fan-made league: Movie (sets 1-10 with King Block sites) is more popular, but KS die-hards say it has too many overpowered cards particularly from sets 8 and 9.