So on the whole I really like the entrechment upgrade, but there are a few issues I've found.
1. It should be very difficult to throw up a starbase in an enemy occupied system. At the very least make it time consuming. I get annoyed when the enemy shows up an starts constructing a starbase as I am attacking or shortly after I've taken over a system. I was thinking it should be based on culture or else simply take a while.
Note: I have no issue with throwing up a starbase around a star regardless of culture. In fact I have to say that throwing up a starbase around the enemies central star has been one of the few bright spots in that it lets me cut an inevitiable game down to a reasonable number of hours.
2. Mines are both bugged and demonstrate a decided need for a minesweeper. - First I see lots of mines that simply don't explode. I've tried rotating the display to make sure my ships were close but for whatever reasont hey did not go off.
Secondly the AI loves suddenly spamming hundreds of mines in a system, completly ringing a central star. This makes it impossible to see what the hell is going on. You need some type of mine sweeper to clear them.
Lastly, i think you should jack up the damage done by mines and their effect range. As they currently stand they are only mildly worrying. I properly mined area should be something that is either in its ability to destroy a complete armada. It should force a bit of careful play.