Glaciation And Great Spring

Dennitza's sun, Zoria, is brighter than Sol and Dennitza is smaller than Earth. Ultraviolet cracking reduced the oceans to covering half the surface. The day is 18.8 hours.

Less than a million years ago, Dennitza was struck either by giant meteoroids or by a shattered asteroid:

on land, impact, concussion, radiation, fire and craters;
at sea, tsunamis destroying the coasts;
in the sky, dust blocking the sun for years and clouds lasting for millennia;
at the poles, growing ice caps;
from the poles, glaciers halfway to the equator;
in the biosphere, mass extinctions, including of a tool-making species, then new forms, hardy or contentious;
later, clearing skies and melting glaciers;
then, storms, floods, more extinctions, migrations, ecological changes and rising sea levels;
meanwhile, human colonists building coastal towns destroyed by rising seas and a capital city in the middle of a large astrobleme.

Note: many interesting and dramatic events occurred before humanity arrived.