I would have to guess the monsters on the train at North Corel.
The Gold Saucer could be considered the tower. The train will destroy the town if you don't save it. On the train are 4 monsters you have to kill, all of which are machines. North Correl itself is the 'base' of a memory for Barret which he clearly remembers very well. If you save the town, you gain the Ultima materia.
It's a strech but that's what I found.