What are the reasons behind this invasion? To be painfully honest, I don't know what isekai means in this context, other than apparently "another world" in Japanese.

I guess I'm just trying to approach the invading army's tactical decisions from that perspective, if you're wondering why I'd ask the question.

As for defending city streets with low ammunition, and manpower, I'd say the best option is to use "turtle maneuvers", be very defensive, avoiding offensive, and attempt to confine and "dose" groups of enemies into tight spaces, then surround from at least three sides (never turn your back to the enemy!) and barrage those tight spaces with arrows and cannons, wearing the enemy down slowly but surely. Thinking about the scene of battle in Minas Tirith, the circular shape and tight corridors should work into your advantage in this. Assuming the invading army doesn't just want to obliterate the city and instead prefers to conquer the city for themselves, they probably won't try to destroy every building or walls on their way, mostly gates and such so that they can advance into the city.

To be honest, if they merely wanted to obliterate the city, they wouldn't advance into the city at all. It just wouldn't make sense.