Learn Italian: I soldati ferirono il nemico al cancello. - The soldiers wounded the enemy at the gate.

Ferirono comes from the regular -ire verb ferire (to wound). To form the "they" (loro) form in the passato remoto (historical past), you simply drop the -ire ending and add -irono.

Ferire takes -isc- in the present tense (feriscono). Why isn't there an -isc- here?

The -isc- pattern only applies to the present indicative, present subjunctive, and imperative tenses. In all other tenses, including the passato remoto, you just attach the endings directly to the normal stem (fer- + -irono).

Why use al cancello instead of another preposition like nel?

To say "at the gate", Italian uses the preposition a (at) combined with the definite article il (the) to make al. The preposition in (which forms nel) usually means physically inside something, whereas a is used for specific points of reference or landmarks.

Can il nemico mean the entire enemy army, or just one person?

It can be both! Just like in English, il nemico can refer to a specific opposing soldier, but it is very commonly used as a singular collective noun to refer to the opposing army or enemy forces as a whole.