Protecting a city

[Dadoman7]

Default City protection
I'm a new dnd player and my dm started my party of three off with the responsibility of protecting a city from a siege any advice is welcomed

[Yahzi]

Without any detail, we can only give you general advice.

You apparently know it will be a siege instead of an assault. Therefore, supplies: you must find a way to feed your city for longer than the besiegers can feed their army.

1. Stockpile food.
2. Find a way to sneak food in (tunnels, boats, or fliers).
3. Attack their food supplies.

[Scheming Wizard]

Heroes of battle has some cool stuff like siege weapons and even magic items that work like mines.

The main thing when defending is to take into account all vectors if attack. There is the air which needs to be defended via ranged attacks like archers on the wall or someone manning a ballista.

Then you have ground level which is defended by the walls and the gates. Make sure the enemy doesn't break down a wall and if they do be ready to defend the breach helms deep style. Also watch out for traitors that open the gate while everyone is asleep. A moat is a good idea. Invest in crocodiles.

Finally you need to watch out for things that can dig under the walls or use tunnels that go under the castle. The best way to keep the underground safe is to collapse tunnels before the enemy can use them to get in. This kind of sucks though if you lose the siege and then need to get out. As far as digging monsters you really need to get down there and fight them. Traps like flooding chambers that get dug into might be a good idea.

There is also teleportation, but you need magical wards to fend that off.