These are the foundation ruins of a building that was underneath one of the former Queenston/Lewiston bridges. It can be accessed by simply driving to the very eastern end of York Road in Niagara-on-the-Lake. Even when the official road ends, keep driving to the east along the small residential street that York Road leads in to. When you cannot drive any further (cement blockades), get out and continue to walk along the pavement (as a side-note, this may even be wheelchair accessible it is so flat). The pavement will end at the ruins, giving you a great view of the Niagara River.
The foundation area is the site of many a bush party so watch for broken glass. Just down the escarpment twenty metres is a walking path that passes close to this location, so no stealth is required exploring this ruin. It is open entry and the Niagara Parks Commission has taken no measures to keep people off the property.
Included a vintage post card in the photo gallery that gives you an idea of what the photos are representing.