I'm stupidly attempting a pirate ship cake for ds1's birthday and I'm using a square tin to cook the cakes in which I'll then cut/shape and stuck together - hopefully 
However I did a test run cake in the square tin for Easter with my trusty sponge recipe and the cake rose loads in the middle but hardly at all at the sides which have it a very odd shape and isn't going to work for making a ship.
I've not had it happen in round tins.
Does anyone know why it did that? What can I do next time to get it to rise evenly?
TIA