Let's estimate the order of magnitude for how many doors are on campus. First, we can bound the problem: we can be sure it's more than 10, and probably even more than 100, right? A million would surely be too many. Let's try to get closer:

How many buildings, order of magnitude (1, 10, 100, 1000), do you suppose there would be?

How many doors would you estimate there would be for a typical PCC building? Count internal doors and entrances and exits, but let's not count storage cabinets and cupboards -- just doors people might walk through.

Let's break that down a bit: estimate the average number of hallways (order of magnitude: 1, 10, 100?), including multiple floors.

Now multiply that by an estimate of the number of doors per hallway (1, 10, 100?)

Enter the total number of doors you get to the nearest power of ten. Is it more than 100 but less than 1 million?