Ok, you asked for comments on the design
I'm imagining how it would move - I imagine that those inward facing 'toes' would be retractable and movement would be on the points of the leg-guards. Maybe it only extends the 'toes' when it needs to be secure to fire that massive cannon?
While it looks great, if that were used in a game I'd request that the cannon be shorter - only because when moving (I'd imagine it would move quite fast) the body might tilt forward and possibly dig that cannon into the ground.
Other that that - may I say you have an excellent eye for 'plausible' designs. Some designs could never work, but some designs make you think that with a bit more technology - they could really be made. And your Arachnoid Tank is a good example - well balanced, weight in the right place, looks chunky and like it could support itself.
