The Roman army was heavily disciplined and fought in tight formations with heavy armor, which allowed them to defeat most of their enemies and keep peace within Rome.
The main advantage of the Roman fighting formation is that there was always someone fighting. Once the people at the front of the formation die or get tired, new ones replace them. This made the army especially hard to break through, since there was always someone fighting.
The Roman government was very important to making Rome a powerful nation. Patricians originally ran the government, but plebians became the only ones allowed to later on. Citizens had the right to vote, and generally voted on most decisions for the country. The people were united.
The plebians were angry that the patricians were the only ones allowed to make decisions regarding their lives, so plebians were made the ones able to vote. Adapting to new situations such as these are one of the reasons their government worked as well as it did.
Rome's alliances in war and trade allowed for little war, so ideas could be spread faster. The Romans would help stop wars their enemies were fighting, as well as granted citizenship to allies.
The least important reason was the writing law system. Laws in Rome were important, but compared to other properties, such as architecture, the laws weren't as important. Most Romans didn't know laws up until they were written, so most people already basically had no laws. Writing them down was the only thing that changed how the things in Rome were.