An armoured car with a tank's entrance hatch and sandbags, falling. Judging from the sandbags, I'm assuming these were for the stronghold in Mission 3, but that's very little to judge off.
Ivo also made a second version with a palette that actually existed in the game.

Information from: http://randomhoohaas.flyingomelette.com/msmia/1/ug.html

PS: Iron Fortress is only a nickname!