Since the reading for 固 (かた) is different than the one you learned with the kanji, here's a mnemonic to help you:
Why would you want to make something hard? Obviously because you want to put it in a catapult (かた). There's no point putting soft things in catapults, after all.
You can harden clay with heat.
Viet takes an hour to make his pompadour hard every morning.
Koichi decided to strengthen his position and send his troops to Japan.