Complex sentence: During that battle the Romans were forced to retreat up hill on the Via Sacra, however, at the Porta Mugonia, Romulus prayed to Jupiter, vowing him a temple should he stem the Sabine advance.
Split into simpler sentences: During that battle the Romans were forced to retreat up hill on the Via Sacra. However, at the Porta Mugonia, Romulus prayed to Jupiter, vowing him a temple should he stem the Sabine advance.