The scale of the city and the scale of the attackers (along with the amount of time available) would be factors in the actions taken. Walls are generally the infrastructure around which resistance to conquest begins. Phillipe Contamine's War in the middle ages mentions that Richard I, on having trouble with besieging a fortification in the late 1100s, inquired with a local monastery and asked them to build an incendiary for him, which they did, and he used to destroy the gates of his target. Accordingly, presuming a siege is forthcoming and known about, having strong walls and stockpiles of food are among minimal requisites for a siege to last beyond the short term.

Konrad Kyeser's 1402 work Bellifortis on siege warfare illustrates and describes many forms of siege and counter sieges devices: hooked ladders for scaling walls, multi-fan devices for spreading smoke, wheeled tunnels for protecting excavators or wall/door breaching work, blankets on ropes to lower down the walls to dull the assault of a battering ram. This shows that even this late, there was still a significant preoccupation with walls as the focus of a siege.

Kyeser built upon the very popular work De Re Militari, which was profusely reproduced in the middle ages. It contains advice on siege craft, logistics of warfare, and general advice on how to act appropriately in a given military situation. It also inspired a work on warfare by Hrabanus Maurus in the mid 800s. Should a town council or lord have had access to a copy of De Re Militari, or someone versed in it, this could have reasonably influenced siege preparations.

Although much past the time period you asked about, the Siege of Vienna in 1529 might also be of interest. Turnbull mentions among other preparations, several of the city's gates were sealed and many modest dwellings along the outside of the city walls were demolished (to remove easily lit material), along with reinforcements made to the walls and earthen reinforcements inside the walls. The pope also made appeals for aid and raised money through large loans. Mercenaries were employed, one of which (Nicholas, Count of Salm) was appointed to be in charge of the defence.

A relevant paper, "Richard I and the Science of War in the Middle Ages"

Turnbull, Stephen. The Ottoman Empire 1326–1699

*edit: minor spelling errors.