http://forum.melee.org/scene-editing/siege-beta-guide-for-mappers/These are the props you will need to make a conquest map. Urist hopefully fixed the flag bugs that conquest had and there will be longtime testing after the next patch. Multi is not implemented correctly yet, so 10 minutes maximum. Make as many layers as you want, although with the time limit i'm guessing 3 layers/ 3 flags is the most you can do. So preferably just scenes with 2 flags.
When multi is successfully implemented (i'll keep you on track if it happens before the next patch, whenever it may come) feel free to make maps that take the full hour. But until then, i suggest you wait for those because without proper multi system there might be a chance that you would be wasting your time with them ;)
Doesn't even have to be one castle to besiege anymore, i'd very much welcome more open maps as well. Just let creativity flow through you, hehe. If you have any questions about conquest, feel free to ask me here or on irc.