The Colorado state constitution specifies annexation rules that municipalities must follow. These are distilled on the City of Littleton's website:
http://www.littletongov.org/comdev/annexation.asp
Which contains the following quote:
CRS defines the procedures and limitations to be allowed. Unless the property is an "enclave” totally surrounded by the City for a period of at least three years, annexation cannot occur without the landowners' permission.