Yeah, every time you retire at level 31 you get an heirloom point and a little more xp per minute on the regular cRPG servers so leveling will be slightly faster each time. You'll feel weak until you get back around level 25-ish but you're now earning crafting points in the weapon/armor that you heirloomed if you happen to have it equipped when you get the ATHW messages, so that'll be good in the long run.
When signing up for the npc side of a castle/town defense, right now the AI hires people with high levels over everyone else, so you might have some trouble getting into those battles until you get back around level 30. Sometimes the AI hires you earlier in the day and then bumps you for someone with a higher level a few hours before the battle too, so be sure and check back. Also, not sure if this is still an issue but leave your requested gold at 0 because the AI used to not hire people that asked for gold, because the AI is a dick.
If signing up for another player's battle, they can see your level so just put info about your build in the application like: 2h 21/18 (strength/agility) 3 riding. Last thing, on c-rpg.net go into the settings/region and make sure your timezone offset matches your current time so you don't show up at the wrong time for a battle.