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The World Trade Center was a building complex which consisted of the Twin Towers (among others). The Twin Towers were destroyed on 11/09/2001 when Islamic terrorists hijacked American airplanes and flew them into the towers. The destruction of the Twin Towers started up some severe Muslim Discrimination:

"You shut your goddamn raghead mouth!" ~Mall Security Guard to Rashim Anwar (City of Shadows)
Because of this enormous and horrible event Waldstein chose this destination and time as the Field Office of the TimeRiders agency so that everyone's attention would be diverted from anything happening at the Archway.
"Because everyone's attention will be on what happened. No one will ever notice the comings and goings from that little archway beneath the bridge." ~Foster explaining the Field Office's location to Liam, Maddy and Sal (TimeRiders
The archway exists within a time buble of forty-eight hours, meaning that the team living there will constantly relive the two days (10/09/2001 and 11/09/2001) in a loop. They constantly re-experience the destruction of the Twin Towers.