One thing that people are frequently taught about Italy is that it has historically been protected by the Alps, which served as a natural barrier separating the peninsula from the rest of Europe. On the other hand, Italy has been invaded countless times by foreign invaders, including Hannibal, the Gauls, Napoleon, France, etc. So how did the Italians benefit geographically from the Alps?
I don't know how comprehensive I can make this because it might be more suited to the "Short answers to simple questions"-format. Anyway.
On a very fundamental level, the alps are mountains, high mountains. They roughly for a crescent around northern Italy. Why is that a good thing? Well, the alps are basically a 5000 meter wall separating Italy from continental europe. Historically this has afforded Italy some protection against invasion for several reasons. First and foremost, the Alps present a natural barrier from invasion. They're simply bloody hard to cross in most places. You simply can not march an army through the alps without much of it dying from exposure and starvation. It also takes a really bloody long time to cross which gives you time to prepare a defense and even if you do succesfully cross it you'll be contained in valleys which often contained one or several castles or other fortifications. Valleys generally confer an advantage to a numerically inferior defending force since a larger attacking force can not utilize more troops than the width of the valley allows.
The second problem with invading through the alps is that even if you get your army across, you have to try to maintain a supply train over the alps which is a logistical nightmare. Most passes were only historically passable in the summer months which leaves you a short window before you might find yourself cut off in some valley in northern Italy with winter closing the passes behind you. Not a situation most commanders would willingly put themselves in.
What about the times it has been invaded though? Well the alps do not cover ALL of northern Italy. The area around Trieste in the east is generally crossable year round and the costal path in the west, while mountainous, is open year round. However, in the west, the route is easily defended and any supply train relying on it would be under constant threat by whoever holds naval supremacy in the region. For this reason the path chosen by both the French and the Carthagians was to cross the alps west of Turin and enter Italy proper in the Po river valley.
They way they did it was broadly speaking similar. Hannibal relied on support from recently conquered and severely repressed tribes in the region to turn against the romans and provide both supplies and troops for his army and Napoleon did it by attacking the Kingdom of Sardinia, forcing them to sue for peace and allowing French troops to use Sardinian bases for supply on their further push into Italy. I know it sounds weird that he attacked the Kingdom of Sardinia but to understand that you have to be aware that in 1796 the Kingdom of Sardinia also owned the region of Piemonte on the Italian mainland. Basically, the alps in the west were controlled by a smaller Kingdom which Napoleon force into a peace treaty that allowed him control over the western Alps.