An often-asked question in history is which country, from any time period, with its current or previous territorial hold and before, during or after its golden age would be the most difficult to invade?
People claim that modern day countries like Luxembourg would be hard to invade, since they are at the very heart of Europe, neighbored by France and Germany. Or perhaps Andorra, the mountainous land locked country bordering Spain and France, home to the sleeping irredentist movements that allow its sovereignty in the first place.
Mongolia would be near impossible to invade, bordering harsh lands, particularly the Gobi Desert to the south. Mongolia also borders China and is shielded by taiga forests and mountains to the north. It is also near Russia; it is landlocked and has a high altitude.
A country like Australia is home to dangerous wildlife with water and shelter being non-existent in those areas. The flora is usually small, dried, withered or bleached and dead from a long, long time. New Zealand, any Micronesian or Polynesian isle would simply be not worth for an invasion, as these isles hold little of value.
Canada?
Most difficult or not feasible at all would perhaps be Canada? Bordering the U.S.A to the south, it has the safest position in the entire world. Protected by the harsh winters and below freezing temperatures, the predators inhabiting the forests and tundra are territorial and fierce. The north itself is protected by the Arctic Circle, perpetual dark or everlasting days; it is a recipe for disaster to anyone giving the slightest attempt of invasion by sea.
Switzerland
Yet perhaps the most often looked over country, would be the one most well-known for its neutrality, Switzerland. The entire country is mountainous, with slippery slopes of sharp edged rocks and summits that reach for the skies. Surrounded by countries like France, Germany and Italy, it has no slight worry about invasion. The entire country is defensible by itself, thus being surrounded by mighty countries only bolsters this fact. Even if that were to fail, most of the bridges around the country are rigged with explosives to further delay an invasion. As conscription is mandatory in Switzerland, the populace has the basic training and discipline alongside survival skills to weather an invasion, and to make matters worse for the invaders, a law is placed that every household should own a gun. Even with all the impressive natural defense, with the capability to withstand and weather any invasion by the people, the help of its allies and mighty neighbor’s falters with the weapon the Swiss do employ masterfully, diplomacy. 
Not having any bad relationship with countries around the world, even if they did not like an action condone by a country, the Swiss prefer to articulate their disapproval by sending a strongly worded letter. Even if diplomacy were to fail them in defending their nation, perhaps the planetary approved keeping of gold and money would be the biggest deterrent of any invasion. Indeed there is no country on Earth that would risk invading the country that houses and keeps safe all their valuables and the Swiss would love to use this as leverage to dissuade any would be invader. That is, if they weren’t so neutral.