There are no lanes, drop gravity wells into open space and curve incoming comets away from your homeworld and into the central star.
This is tower defense without fixed paths. Waves of comets streak toward your homeworld, and your defenses are gravitational masses that warp their trajectories. Place wells, anchors and other structures during the build phase, start the wave, and watch the swarm bend off course. Destroyed enemies pay salvage, which you reinvest to upgrade your masses or add guns, mines, patrol ships and support.
Tower defence rests on a path: enemies follow a lane, and towers are placed along it.
Here there is no lane. Comets fall toward your homeworld and your defences are masses that bend their trajectories, so you are not blocking a route — you are creating one.
This is the same physics real missions use. A gravity assist redirects a spacecraft by flying it past a planet, changing course without spending fuel. Making that the defence inverts the genre: a well is not a thing that stops a comet but a thing that decides where it goes.
Orbital Siege is an original free browser game that reimagines the tower-defense genre with open-field physics instead of set paths, defending a homeworld with pure gravity. Built in-house for PlayPendium, it plays entirely in your browser.