While Carthage had been advancing from the south Rome had been pressing forward from the east along the shores of the Mediterranean, and had planted herself firmly at Marseilles, a port which gave her a foothold in Gaul, and formed a base whence she could act in Spain.

In order to check the rising power of the Carthaginians there she had entered into a firm alliance with the Saguntines, whose country occupied what is now the district of Valencia.
By the terms of the last treaty between the two republics each was forbidden to make war upon tribes in alliance with their rivals, and Saguntum being thus under the jurisdiction of Rome, an attack upon it would be almost equivalent to a declaration of war.
The position of the city was one of great strength.

It stood on an almost isolated rock at the foot of a spur of the mountains which formed an amphitheatre behind it.

Around it extended a rich and fertile country, the sea was less than a mile from its walls, and the Romans could thus quickly send succour to their allies.

The rock on which the town stood was well nigh inaccessible, falling sheer down from the foot of the walls, and was assailable only on the western side, where the rocks sloped gradually down to the plain.