You might remember the meaning of the character 域 with this easy mnemonic:
In our region, you can see people using earth (土) to rub their halberds (戈) so they don't rust. It's an old remedy passed down by mouth (口), and is one (一) of the many ways we maintain our weapons.
Using mnemonics to remember the meaning of Chinese characters like has a huge impact on how long to learn Mandarin it takes.