Hmm, The easiest towns to take and hold are actually Barriye, Khudan, Praven, Yalen and Dhirim. ( From the easiest to the most difficult).
Barriye is the sarranid city at the edge of the map, with three villages surrounding it for quick replenishment of youre garrison, and due to the desert and the big distance between it and the other sarranid towns ( Ahmerrad being at one day and a half for most army's), it is by far the easiest one to hold.
Khudan can also prove to be a good city, at the heart of the vaegir lands, the pesence of a lot of tundra bandits surrounding it will usually make flee most lonely lords. Its presence inside of the vaegir kingdom will make most of the other factions think twice before attacking you, and even if they do so, they will usually end up journeying for over three to four days in average, giving you enough time to prepare. However, and if you are to take it, you will need to turn some of youre companions into lords right away and give them the surrounding villages, to prepare for a quick invasion of vaegir castles surrounding it.
Praven is usually tought of as a hard to hold city for it being surrounded by swadian castles and being near Suno... Actually, Swadians are really bad at siege assaults so you will easily get rid of their first attacking army, after that; you just need to march for the neighbouring castles ( Tevarin and Ryibelet), so as to have a nice base right inside enemy lands.
As for Yalen, it is trully hard to take ( Rhodok castles and cities are always hard to take), however, this can also proove to be at youre own advantage, as you will find it quiet easy to protect, I wouldnt recomend you to go for it tought, as surrounding castle are trully hard to take and rhodoks are just way too good in siege battle.
Finally Dhirim is by far the easiest to take as it is usually the object of a lot of siege battles, and thus never having a strong enough garrison ( I even saw it change hands between four factions in less than two days). Its position ( Right at the middle of the map), makes it quiet the rich town, however, it will also turn into the most difficult to hold and you usually end up in war with ALL the other factions. If you are to go for it, leave it as soon as you took it and march for Senuzgda, Derchios, Reindi, Kelredan and Tilbaut castle, if you have a good engineer, you shall be able to take at least Senuzgda and Derchios before the enemy retaliate, give them to companions and prepare to defend against... A lot of enemies.
Seeing as you use mostly Nord troops, I dont think you will find problems in taking any town, however, holding them against all the factions will be another story. I would recommend you to take Barriye first, recruit nords and rhodoks en masse ( easier done by using the diplomacy recruitement system), and train them into sharpshooters and Huskarls. A good 250 top tier garrison shall be enough in the begining, with a 40% infantry/ 60% archers ratio.
By the way, dont begin youre attack unless youre enemy is already at war against one or more powerfull factions.
Hope this can be of help.