"Useful," Solomon said thoughtfully. A trap like that sounded like one of the more useful things he could want. "Are there limits to placing such a trap, either in terms of location or lifetime?"

Did it have to be put into the ground, or in a building or door? If it could be specifically localised--to objects, like a desk or even a pocket-watch--it was the sort of trap that would be most effective laid far in advance and carried around.