Were the Saturn V(s) assembled in the Vehicle Assembly Building (VAB), on the Crawler transporter or were they assembled on the ground and then kept over the transporter to be taken to the launch pad? If they was assembled on the transporter, was the transporter in a "trench" - meaning the surface on which Saturn V would sit, was at ground level?
This is covered well in the Saturn V News Reference Vehicle Assembly and Launch.
Saturn V stages go into the Vehicle Assembly Building (VAB)...
The (first) stage is positioned above the launch platform of the Mobile Launcher and lowered into place.
The upper stages are prepared for mating...The mating operation consists of stacking the stages.
When the launch vehicle is ready, the Apollo spacecraft is brought to the VAB and mated.
At this point the Mobile Launcher is not yet mounted on the crawler. After the successful completion of testing on the stacked vehicle,
...the transporter is moved into position below the mobile launcher. Hydraulic jacks engage the fittings on the mobile launcher and raise it approximately 3 feet so that it clears its mount mechanisms. Then the transporter moves out of the VAB...
For details, see the link.
This picture shows a first stage being lifted up by the cranes.
(NASA photo via popsci.com)
No trench, the crawler / mobile launcher drove into the VAB on its floor level and the stages were lowered from above with cranes. This is a shuttle picture but you can see the crawler leaving the VAB at floor level.
(NASA photo via wikipedia)
The Saturn V is stacked on the "Mobile Launcher", but it is important to realise that the "Mobile Launcher" is, by itself, not mobile. As you can see in this contemporary photo from when the Saturn mobile launchers were built, the mobile launcher is only a platform with the launch tower:
(NASA via here)
It is moved by the crawler, which moves under it (note that the platform has "legs" raising it from the ground). The crawler itself has a flat top surface and just slides under it:
The stacking process is outlined in this flow chart:
The relevant items are:
- L/UT [Launch/Umbilical Tower] is moved into high bay
- Booster [S-IC] transferred to erection bay & erected on L/UT
Steps 18, 19 and 20 stack the S-II and S-IVB stages and the instrument unit.