Corporate flight crew reported low altitude alert on approach.

Narrative

Anticipating visual XX ZZZ as reported in ATIS we went to select on FMS. As it was not in database we built the approach with waypoint and altitudes while still in cruise phase. Pilot flying (PF) and pilot not flying (PNF) cross checked points and altitudes. Once cleared flew it on Autopilot (AP) as LNAV/VNAV to last visual point and altitude prior to landing. On approach Tower called a low altitude alert and I immediately disconnected the AP and climbed and cross checked the fix altitude. It turned out when we built the approach we failed to repeat an altitude that appeared twice and put in the next lower altitude one fix prior to where it should have been and were subsequently ~400 low when the Tower called the altitude. After correcting back to altitude we flew rest of the approach uneventfully. On taxi in Tower asked us what happened and admitted the mistake and they said thanks; did not instruct us to call anyone. On debrief we realized what we had done and were surprised despite building the approach as a crew and cross checking and verifying we both missed the repeated crossing restriction. This error could have been prevented if we had a programmed FMS visual approach for this arrival; but lacking that it under scores the need to check; double check; and check again any procedure that you manually construct.