The good thing about 2fort is that there are so many options for engineers, placing your sentry on the intelligence is... not a great idea. You're not covering any ground of the map, you're giving up ammo and health to the enemy and you're just begging for a C&D stab and sap.
Just consider the other building opportunities this map offers:
1. By the courtyard spawn
By the spawn, covers most of the courtyard plus the battlements. It can be edged easily however, so consider placing dispenser strategically. A dispender placed properly in the battlements can block scouts, absorb damage from spam classes and still not give enemies sufficient cover. Remember: if you can't sneak past a dispenser so can't your enemies.
2 and 3. Over the courtyard
These placements support the placement 1 by making it harder to edge it and by targeting a spy going for a sap and stab.
4. By the intelligence, but not over it
Guards directly the intelligence without sitting over it. Guards the healthpack and the ammo pack that otherwise would be used against you. Intelligence carriers can't avoid it, but it can be easily edged from the stairs. Last line of defense.
5. By the snipers
If you're getting overrun by scouts, this will surprise them a couple of times. Prevents snipers by getting stabbed by giving them a better target: yourself. It won't last long. Consider using minisentries while tanking other sentry placements such as 1.
6. On the enemy side of the bridge
A surprisingly strong placement that doesn't actually defend anything but helps your team go on the offense. Just keep in mind you're directly below the red far spawn.
7. In the enemy sewers
A popular placement for offensive engineers. Make sure you plan well your metal expenses, or your base will be destroyed easily. Upgrade your teleport entrance to level 3 and haul a sentry or a dispenser in position if you can. Once you're there you only have so much metal. Your dispenser going down pretty much is a death sentence for your base here.
The favourite placement of combat engineering, the water and the knockback makes it considerably harder for weaker classes to target minisentries while making you fireproof.
9. At the upper entrance T-junction
Certainly another annoying placement close by the spawns, it covers all but the path over the roof, which many classes can't take anyway (especially medics). You might have trouble setting up however: the Eureka Effect might help.
10. By the courtyard
Another support position, it targets enemies edging placements 1-3 while also covering that room, a popular place for the enemy to hang around in to build uber. Watch out before you get edged yourself!