When the root ends in ns, miss, or soft c or g, use -ible.
Take the word defense as your example - as you can see here, the root word ends in -ns, and is then followed by an -e. In order to make an adjective out of this noun, you need to add the appropriate suffix, which in this case is -ible: defensible. This refers to words whose roots end in miss, and soft c or g as well. The only exception to the rule is the word indispensable, where you add -able instead of -ible.