Mount & Blade II: Bannerlord

**I unlocked this achievement by capturing a castle on my own which potentially changes the meaning of the achievement description (To a town or castle). If anyone else can confirm this on their game then this achievement becomes much easier. Also, im fairly certain an allied army joined my camp after I initiated my siege. I'm not sure if me laying siege before they joined was why I got the achievement or possibly, none of their troops ever needed to be deployed during the siege maybe? Again, if anyone can confirm that would be great.**

To earn this achievement, you must besiege and capture a location using just your own forces (not part of a combined army). I would say it is best to have at least the capacity for 100+ trained troops for the below method, however if you are to attack a castle at its full strength, you would need an army of over 250+ trained soldiers as castle typically range from 200-300 garrisoned troops under default conditions.

The best case scenario to look for is a castle on the border between two nations at war that gets captured multiple times in a short space of time by each side. By this point, the garrison left at the castle can be around 20-40 men and the fortifications are likely ruined. Make sure the roaming armies are off capturing other fortifications or at least out of sight and you should be free and clear to capture the castle.

To besiege a castle, highlight and click 'A' to interact with the castle, then chose the option to lay siege. Your army should set up camp. (At this point create a new save, just incase a large army comes back to defend the castle, if you're early game you can't really afford to lose troops so just reload and break your seige then allow more time to pass. Hopefully the enemy army won't be close enough on your next attempt.) Click on all 3 boxes at the back of the camp to build 2 siege towers and a battering ram. Once these are built, build 4 trebuchets in the 4 boxes in front of your camp. Queue all 4 at the same time and as each one completes quickly pause the game (using the 'X' button) and click it and put it in reserve. Once the last one has built pause again and drop all 4 pre-built trebuchets at the same time in each slot - Do this because it means the enemy defence won't wipe out each trebuchet as you build it. Resume play and watch as the enemy siege weapons get destroyed. They will immediately start to rebuild them, so once enough are destroyed and hopefully before any new ones can be built (find a happy medium as this is fairly random), pause your game and save in a new file slot (just incase you fail to capture the castle) then, while still paused chose how to start your siege of the castle. You can now go in with your troops or use the auto-resolve feature.
Once you finish the battle (and win) click off the battle report and the achievement should pop.

To get this achievement you need to take a town with your party. Wait until the enemy faction takes a town when they leave take it as it won't have too many enemies there to protect it. Have a least 100 in your party. I hope this helps.