24. Castles are the most cost effective way to increase your feudal levies, since they have the greatest variety of military buildings and investment there increases troop strength more than other holdinigs. Some strucutures increase fort level, some increase the levy overall (proportionately across troop types), and some increase specific troop types by the stated numbers of men (which are multiplied by those overall % boosts from other buildings). Troop selection and cost per man is higher in other types of holdings. Check the terrain map for terrain modifiers for combat and focus your troops accordingly (some light cavalry is good for pursuit - knights require several levels of upgraded stable).
However, investing in income-producing structures gives higher returns in towns than castles, and there is more variety in available structures. If you want a structure that increases technology development, you can build that in a bishopric.
25. The wrong type of demesne penalty applies when a feudal lord direclty holds a bishopric or town, and is a substantial income penalty. It may, however, be worth it if the alternative is a disloyal local vassal who won't pass along much income or release many troops for feudal service, and can attack you when your back is turned. Better is to get your vassals to love you, so they pay appropriate taxes and provide adequate levies in war - giving them titles, cash or land, arranging marriages, satifying ambitions, virtuous acts (like releasing prisoners), and other events can help improve the relationship, but study their opinion tooltip first to see if there are particular obstacles to a good relationship.
26. Rulers have 5 stats, and the parenthesized numbers on the right are the total state stats in that area (effectively boosting the ruler's performance). State stats equal the ruler's stat + 1/2 (rounded down) of the spouse's stat + the relevant minister's stat (e.g., Marshal's martial stat). Women can't serve on the council other than as Spymaster if wife or mother or the ruler, but their skills and counsel definitely do make a difference.
P.S. Don't just bump this - add tips!