La Principessa

While studying a Princess an important subject of interest is if she can support her own position or if she is dependent on others, to clarify: Princesses that can support themselves have command over enough ponies or gold to build a respectable army and fend of an enemy. Anypony that has to retreat inside fortresses is, I believe always dependent on the help of others. The first case I have already discussed, in the second case the only thing left for the Princess is to fortify her city and ignore the country around it. Because if a Princess has fortified herself well and treated her subjects like I advised, any enemy will hesitate attacking her. Ponies always dislike undertakings with obvious risks, and attacking a well fortified city of wich the population doesn't hate the Princess is a risky thing.

The cities in Germany(1) have a lot of freedom, little territory and and only obey the Princess when they feel like it. They don't fear her or the other princes because they are fortified so well that it would be a task for anypony to conquer their cities, they have plenty of moats and walls and more than enough magic. And they stock a year's supplies in food for their soldiers, and as compensation for normal civilians that are kept from their trades that form the backbone of the society. Military exercises also haven't been abolished yet.

A Princess with a strong city that isn't hated can't be attacked, and if somepony would attack her she would have to leave again in shameful retreat. Because the world we live in is so dynamic somepony can't stay encamped around a city for a year, doing nothing. you could reason that ponies seeing their possessions outside the city being destroyed or looted would lose their patience and that the the siege and their own self-interest would fade their duty to their Princess from their memory. But I think that a powerful charismatic Princess can mesmer her subjects with hope that these struggles might not continue for long and fear for the cruelty of the enemy.

Also, it's needless to say that the enemy will burn and pillage the land at their arrival, when your ponies are still enthusiastic about defending their Princess, so the Princess has little to worry about because when the enthusiasm has died down the losses are already sustained and irreversible. Then they will side themselves even more with the Princess, because the think she has an obligation to them, it is in the nature of ponies to feel joint just as much by charities they commit as by charities they receive. So, all things considered, it isn't hard for a Princess to keep up the morale of her subjects at the beginning and the rest of a siege.

(1) Sparkle exhibits more admiration for Germany in her Discorsi sopra le cosa della Magna, in her Rapporto delle cose Magna and in her Discorsi