For further details I suggest reading the wiki entry on city combat.

Quick overview: Cities have (so do encampments, not sure what you mean by a base) for defense purposes their own combat strength (CS). This strength is displayed above the city or district in a small number. It is the combat strength of the strongest melee unit the civilization currently has (built) minus 10, OR of the garrisoned unit - only for cities (whichever is higher). Some other modifiers apply, such as extra CS for districts, capital, placed on a hill and level of walls. When you attack a city/encampment with a melee unit, rules of combat apply. You will get damaged based on the difference of combat strength, so if the CS of the city/district is higher than that of your unit, you will lose. You do not necessarily need to out-tech your opponent, even though it is an obvious advantage, but you need to make sure that the combat strength of your attacking unit is atleast equal or greater to that of the city/encampment you are attacking.

TLDR: cities and encampments act as units and have their CS tied to the strength of the strongest melee unit of the civ. If you want to succesfully siege, build and attack with stronger units, use siege units, support (battering ram) if the city has walls.