How do you capture a castle?

You just have to move close to the castle and attack. I highly advise you to recruit an entirely elite army. (For a start you just have to stroll around the towns to recruit them with a low price or possibly even free!) (Walk to a castle/town, press Town, press Stroll around the city) By doing so you will have a guaranteed advantage over the enemy. (My best record is 0 dead to 100+ enemies killed) My other tip for you is, USE SIEGE TOWERS ONLY. Catapults are only good for destroying the enemy archer tower defences (Never underestimate archer towers. They are your top priority to destroy as they are the ultimate defence against your troops and can wipe out a huge amount of troops in a short amount of time. 5-6 hundred of my troops prevailed against close to 2000 troops thanks to the archer towers.) Battering rams serve as a good distraction for the catapults that are aiming at your siege towers. To know what are the enemy's defences, attack first. Then zoom in to view their defences. If you spotted catapults and/or archer towers retreat immediately and attack again, but this time with battering ram and/or catapults. I also recommend you to use bows and crossbows while attacking cities as engaging in melees are not worth it. You will die faster. Only use the melee weapons if you are confident you can survive long enough to inflict considerable kills and help your troops tank incoming damage. Good luck!