hi colin,

you are correct in that every page in your site is a landing page

you might be thinking of the older name for this strategy 'doorway pages' which historically were not implemented well and did and still do get sites banned (BMW)

Landing pages (keyword target pages, information pages, seo page, etc) if implemented well is a perfectl legitimate strategy. The OP would not use different domains, the pages would sit on the one domain (preferable at the root) and they are built and written to target a specific phrase. 'london sightseeing', 'sightseeing in london', 'things to do in london', 'cruises from london', etc.

Of course all pages in your site should be optimised for a specific phrase or set of keywords but in competitive markets it is necessary to create more content for your site to reach a wider audience.

Those links incrediblehelp provided should help you understand more.