War Sickness

If you are not allied with or part of the participating towns/nations and you enter a siege zone during a battle session, you will get war sickness. This is to prevent third-party interference. You also need a military rank to avoid siege sickness. If you only have a town, then the mayor, co-mayor, or assistant should assign the Guard rank. If you're in a nation, then your king, co-king, or assistant should assign the Private rank.

Town Military Ranks:

• Guard

Nation Military Ranks:

• Private
• Sergeant
• Lieutenant
• Captain
• Colonel
• General

You can also pay your soldiers with /sw nation paysoldiers <amount>.
Read more at the SiegeWar Guide.