Level 10 is the only level in the campaign where you quickly have to face a rival keeper, as there are no walls here. You really should be stronger than him by the time he attacks,... perhaps build more imps and small treasure rooms to more quickly mine your own gold. You know that the bridge always comes from the south, so build your own training room near there so you won't be caught unaware. If you have the bigger army, there are no issues right? Kill everything and capture his creatures instead of having him take yours.
Lvl 11 is heroes only. Lvl 12 actually has an issue in the 0.4.6. release where the keepers can take over the center. If you don't want that, update to 0.4.6u. Lvl 13 will have the keepers more quickly take over the center as well, but you have gems so you should be fine. lvl14 you can wall out the keepers and just mine the gems until you are full strength. Lvl15 you'll never really get attacked and the keepers will more or less die from the heroes. Lvl16 the heroes are most problematic, lvl17 and 18 just have heroes. Lvl19 is one of the easiest levels in the game, the keeper will never attack you. Lvl 20 is against the avatar, but in 0.4.6 you'll have to kill blue as well. In 0.4.6u this is restored to how it originally was.