I discovered this combo in a game today. This probably isn't a combo you're likely to see that often, but it works very well when these two cards are present.
The main problem with Death Cart is that it is difficult to line up the Death Cart with your Ruins. You rely on shuffle luck too much, and the Ruins clog up your deck in the meantime. Having a Watchtower in hand when you buy Death Cart eliminates this problem: simply trash one Ruins, then topdeck Death Cart and the other Ruins. This effectively turns Death Cart into a guaranteed two-shot terminal Platinum with no penalty, which can help you jump to expensive cards quickly.
To pull off this combo, you only need to have a Watchtower and $4 worth of treasure in your hand, which is not very difficult. You will probably want more than one Watchtower in your deck so you can do this repeatedly.