I propse this ...
When you sign up to seige a castle you must pay the sign up guy your 300 gems for a seige headquaters. Once you pay him and are signed up you recieve a untradeable undroppable item that allows you to summon your headquaters.
That item will only be good for that one seige and if you do not use it, it disappears after the seige is over.
You can buy more then one and can buy more during the seige if you need to. But you must purchase one to be signed up.
Or the more devious of those ...
1) If you sign up and fail. Town guards will Kill you on sight for one week.
2) During the seige period anyone signed up to seige receives 0 xp 0sp 0 drops until the seige is over.