Every siege was different because local conditions were different. This depended upon geography, supply lines, the number of besiegers and besieged, equipment available, local terrain and probably other things.

Yet there are some similarities. The Romans were masters at fortifications. If you read through Julius Caesar’s Gallic War or Civil Wars you could sometimes accidently think he was talking about WWI trench warfare. In fact, these two books are goldmines of first-hand information about sieges and you should check them out. In the Loeb editions they also include (sometimes speculative) maps of sieges and illustrations of siege warfare. Most of my information comes from Caesar.

The besieger camp was typically laid out just as the Romans normally did: like little walled towns. These normally included at least two main streets intersecting in the middle and were orderly places. Here is an earlier question about camps you might find interesting.

Normally the camp would be separate from the earthworks, but I believe in some cases it was actually integrated in the besieging fortifications, such as when Caesar briefly besieged Pompey at Brundisium.

Nearby the army would encircle the besieged with earthworks. Where possible, this would be supplemented by palisades and towers. In Civil Wars is an episode of the siege of Massilia, where both parties kept continually building higher and higher so the other didn’t get a height advantage. It was dirty, gruelling work.

And work the soldiers did. Normally, they didn’t just sit back and wait. There was all the normal life of the camp which had to be taken care of – cooking (which most individual soldiers did themselves), repairing equipment, finding food (Caesar talks about this a lot: they normally had to get their food from the local inhabitants), digging wells if necessary, and the like.

But the main non-fighting job was working on the fortifications. They didn’t simply dig a trench and sit in it. They were constantly enlarging them, moving them, extending them or strengthening them.

Sometimes they tried to dig under the besieged town’s walls to undermine their foundations and they would collapse.

Then they also had to build all the equipment, like siege towers, covered mantlets, curved galleries, wickerwork screens and weapons like the ballista or catapult. Expert engineers and architects helped oversee the more difficult and complicated work.

At times sorties were made from the besieged or attempts to storm the walls were made by the besiegers.

One of the most famous incidents was Caesar at Alesia. He was besieging the town, but a relief force of Gauls forced him into the defensive. He basically had to besiege a town while he was himself being besieged. He built two rings of earthworks and fortifications: one to keep the town invested and one to protect his own army from the relief force.

You get the impression that sieges were hard, unpleasant work. Soldiers didn’t seem to like them much and would rather fight a pitched battle than spend all day digging trenches and hewing logs.