Using the RC mod, you can set the map to waves mode and the titans will automatically head towards the players, who would presumably be holed up near the fortress. As long as your map isn't huge, then it should seem more or less like the titans are headed for the fortress.
If you don't want waves per se, you can set the Titan Spawn Cap to a number >1 and put in endless titan spawners to keep the number of titans on the map from reaching 0 and advancing the waves. Keep in mind that a high max number will make for laggy maps and that if there are enough players it is still possible to advance the wave by killing all the titans on the map within a couple seconds of each other.