Edit: more difficult in 3.3.0a, but still possible.
Although it looks impossible, sieges are able to climb the low walls of the corner towers that make up the fortress. This bug is interesting because it worked since the expansion was released, but Blizzard changed the hitboxes of sieges, making it no longer functional. A side effect to this was Demolishers could pass through narrow doorways. When they fixed the narrow hitbox issue, this bug reappeared and currently exist in the game (3.1.2).
Place your siege vehicle next to the low wall and turn it sideways so it is parallel with it. Move in a forward direction by the low wall and in a straight line, trying to make your vehicle brush up against a corner as you pass by. Don't curve around the wall, keep your vehicle moving in a straight line. If you pass by the wall, turn 180 degrees and brush up against the wall in the other direction.
If at any time you notice your wheels spin at full speed, but you don't move forward very fast, this is a good sign. When this happens you are climbing upward slowly. When it looks like you gained some height but are running out of wall in front of you, turn 180 degrees in place and go forward in the opposite direction.
After 2 or 3 passes you should be able to turn your siege toward the tower and enter the fortress.
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