It's useful, I think, to first differentiate fortifications into three basic types. These are the fortified town, the baronial castle, and the ducal/royal castle. The three really only have in common the fact that they are fortified places of strategic importance; the way in which they are constructed, manned, and function in peace and war differ dramatically. I want to make this very clear: everything I speak about below applies to 11th-13th century England and France, and should not be taken as definitive in regards to any other area or period.

The fortified town is just that - a town which has been reinforced with walls of some variety. These could be permanent fortifications, ranging from masonry walls reinforced with towers and gatehouses at the high end of the scale, to simple timber palisades at the low end. The penultimate fortified town of medieval Europe was probably Constantintople, which was defended by some twelve miles of walls, four on the land face, and eight on the sea face. Some had no permanent walls, but in times of crisis were hurriedly encircled with jury-rigged ramparts of earth, timber, and stone, sometimes in as little as two weeks. Regardless of their scale and sophistication, fortified towns suffered from the same problem: they were not purpose-built military installations, but population centers around which defenses had been thrown up. Generally, this meant a weaker geographical position, many entrances which must be defended, and a long circuit of walls. The longer the stretch of walls, the more men are needed to defend them. Though it may seem counter-intuitive, a city with a garrison of 500 men may well be weaker than a castle with 50, because those 500 are less concentrated. A small or unimportant town might be forced to rely wholly on a local militia of artisans and merchants; a more important city, like Rouen or Paris or Poitiers, might have the benefit of a permanent royal, ducal, or comital garrison.

The second fortification we will talk about is the baronial castle, though that term is unfortunately imprecise. These are fortifications of local, rather than regional, importance, belonging to mid-level aristocrats referred to variously as castellans, barons, vavasours, and lords; the regionally, but not nationally powerful men. The majority of castles were of this type, and they could be very numerous, with eight, ten, or more in a given county or shire, spaced every few miles, this being the area which a small mounted garrison could exert control over. This is a dedicated military fortification, generally sited on a site of strong natural strength - most commonly, a ridge, hill, or man-made mound called a motte. It varied with time and military technology, but could range from a simple wooden hall surrounded by one or more timber palisades, to a stone keep (a very large tower) surrounded by palisades or stone curtain walls, to, in the most lavish cases, a series of two or three interconnected stone courtyards, with towers and gatehouses. But it was also an aristocrat's court, where a lord and his household of knights and servants lived (though it was not uncommon for a magnate to have two or even three such castles, in which case those not resided in would be more purely military structures). In event of siege, it would be defended by the aforementioned household, supplemented by landed knights owing service to the lord and the free peasants of the nearby area. These castles, owing to their small size, inadequate garrisons, and lack of stockpiled provisions, tended to fall very quickly to determined besiegers.

The third variety is the royal or ducal castle. This is more what people think of when the term castle is mentioned. Besides being generally larger and more modern than the typical baronial castle, it is a fort, not a home, and almost always defended by a permanent, often quite large, garrison under a commander appointed by and answerable to the king or duke. It was very frequently constructed at great expense and with great care, almost always to defend a strategic point, such as a river crossing, an imposing height, or a threatened frontier. Thus we can say that these castles, rather than being locally important, were very often regionally important; their possession or loss determined the possession or loss of the wider region. With the funds of the crown to back them, these generally were kept very prepared for siege, with huge stocks of food; Chateau Gaillard survived 7 months of siege before being forced to surrender, not by lack of food, but by the compromising of its last remaining defensive wall.

The immediate defensive measures were quite similar among them all. Get in as much food and drink as possible; reinforce any weaknesses in the walls, often with wooden hoardings; get every able-bodied freeman available under arms. Then get ready to wait for as long as it takes a relieving army to arrive, the food to run out, or the defenses to be breached and surrender compelled.