Complex sentence: The Aetolians set up an honorary stele on a base which presumably depicts pieces of armoury of the Gauls, whereas they also erected the so-called "Portico of the Aetolians" or Western Portico, one of the largest buildings close to the sanctuary of Apollo.
Split into simpler sentences: The Aetolians set up an honorary stele on a base which presumably depicted pieces of armour from the Gauls. They also erected the so-called "Portico of the Aetolians" or Western Portico, one of the largest buildings close to the sanctuary of Apollo.