It highly depends on a year you are on, capabilities of your troops and the map itself.
Early game forts are much easier to take over than late game. At the start getting the troops might be hard since you only have access to insurgents.

• Get your economy running, its much easier to gather an army if you have money, take hostile actions on neutral villages (you dont risk war, yet) or enlist in other empire army. Invest in high income factories that wont bankrupt fast (like paperworks)
  
• Get higher leadership/troop limit, farm renown on neutral patrols, you will lose a lot of honor but you can get some xp and renown. Another trick ist enlist yourself and get 7 rank while serving. It will boost your troop limit almost twice so you can get more soldiers and then retire, keeping all of them.
  
• Arm yourself, get a repeating rifle or lever action rifle and a good sword, you are aiming for fastest one with a bit of length. 108/100 is a good choice.

If you plan to take over a fort make sure the faction you want to attack is weak enough, Some factions have a lot more generals than others which will make it harder.
Some faction have a lot of land where cities are far enough that you can besiege fort before they can make it, this makes it easier.

For the fight itself its a bit tricky. If you have number advantage, lets say 90 to their 77 you can risk shortening the distance and forcing your army to use melee weapons and charge. Enemy AI will try to shoot you down, taking a long time for each reload. Your bayonet charge will finish them off very quickly, it usually takes one hit to kill a troop (thats why you need sword).

If numbers are more even you will have to do a lot of work yourself. Move your army to a hill and use it like a trench. Order them to line and wait untill they clear out trench a bit. Then you can move closer yourself and use your clip rifle to kill tropps on tower or leftovers in trenches. Once you get number advantage force melee and charge.


If you already have custom faction and want to storm more settlements you can cheese it a bit.
When customising your troops make sure to use lever action rifle like Henry and sword (the first one on page 600/700 is good enough). Upgrade pistol skill to the max (it sometimes needs weapon master) . You want iron flesh and athletics too (first ironflesh, then athletics, all spare points should go to strength). You can set some higher levels of that troop in tree as Guards. Get as many as you can and with this composition storm castle. You will have to form two layered line and quickly get as close to first trench as possible, use U view and place them in front of it. Order them to hold fire and wait untill they get close enough. Then let them fire and wait untill all enemies die. Repeat it for every trench. Remember to take breaks for stamina and discipline to regen.