Without rambling on for half an hour I've personally found the use of motion sensors attached to an audible alarm is becoming more prevalent around many constructions. Especially but not limited to CBD based ones. I feel as if in many cases that deterrent is enough. Most people hear an alarm going off and they'll leave. Sometimes these also attract some form of physical security to come and check the site. They're not usually super fast in getting there.
I've seen fake cameras used on a few sites. Visible deterrent I suppose. Not very effective.
And of course a select few sites have security posted onsite. I've only encountered this twice however. Sometimes it's still workable. Sometimes not. Depends how diligent your seccy is.
By design the staircase is often towards the core but not always. It is usually practical to get into it as soon as possible. However as stated many sites have motion alarms, these alarms will only be present between usually the ground floor perhaps as high as the 2nd or 3rd. In many cases circumventing these levels altogether by climbing the outside of the construction via scaffolding, the crane or other such means is a good idea.
Wander often. Wonder always.