DefinitionThe most skilled men of our town serve at the workshop. They improve our troops and warships equipment with the latest inventions. May they become even better and stronger! Every stage of improvement allows you to get more upgrades for troops and ships.
EffectThe workshop applies upgrades to all existing units and to any you will build in the future once the research is complete.
All effects occur on all islands whether there is a workshop in that colony or not. So really its only worth having one at any given time as no matter how many you have you can only upgrade one thing at a time anyways. The crystal island would be the most suitable base for a workshop, as all upgrades require gold and crystal.
Finally you get to keep all the upgrades you have researched even if you demolish the workshop.Thus the building kinda becomes useless once you have done all the research.
A small tipCurrently you only need to upgrade your workshop to level 24 in order to be able to research all the available improvements
for more infohttp://ikariam.wikia.com/wiki/Workshop
hope this helps