Giff*, but yeah, with a strength score of 10, according to RAW, you could lift that. If it exceeds your carrying capacity - on its own, not counting your actual carried gear - your speed would drop to 5 feet, but if you have 20+ strength you could push and drag it without a movement penalty.

That's not the same as using it as an actual shield in combat, however - that would require you to add it to your carrying capacity, which would give you no additional weight left for carrying anything else unless your strength is above the natural limit of 20. That said, as a DM I'd rule you'd be able to use it as cover - three-quarters cover, most likely - with the caveat that it's directional (where a normal shield gives you its bonus no matter what direction you're being attacked from), and moving it would take your action in addition to movement.

Of course, that's all assuming the thing is 300 lbs or less. Steel weighs about 490 lbs per cubic foot. Most shields run about a quarter to a half an inch thick. I can't imagine the thing needing to be more than 4' wide, and at 7' tall, a quarter-inch-thick tower shield would come out to about 285 lbs. Any thicker or wider and it'd likely break past your carrying capacity, but you could still hold and move it - it's just drop your speed to 5 when doing so (unless it weighs more than double your carrying capacity, then you couldn't lift it at all).