An invasion is a GameMaster-controlled surge of creatures usually in a town or immediately surrounding area. Invasions are often connected to storylines or the traditional holidays (e.g. turkeys in Wehnimer's Landing on Thanksgiving). During an invasion the justice system will usually be disabled, allowing for more dangerous spells to be used in town.
Character death is typically part of an invasion. Starting in the late 2000s, GMs began to set up invasion zones catered to different levels so that a many players can get involved, even announcing in advance on the message boards where the zones would be. These zones do not always happen and there is no guarantee that a high level creature will not wander into a low level zone.
Wehnimer's Landing, Ta'Vaalor, and Icemule Trace are all set up so that lower level characters can participate by fighting from the rooftops of buildings and/or ballista towers. See here and here for more details of how the rooftops and towers can be accessed.