Cologne - While the Bundesliga's many stars continue to build up their fitness levels ahead of the new season, clubs up and down the country are working tirelessly behind the scenes to strengthen their playing staff in time for the big kick-off in mid August.

Over the next few weeks, bundesliga.com will be taking a closer look at each of the 18 top-flight clubs' summer transfer dealings so far. Next up in the series: Borussia Dortmund.

How they fared in 2014/15...

Last season was memorable for Dortmund fans, but for all the wrong reasons. An ignominious 2-0 defeat to Bayer 04 Leverkusen at home on Matchday 1 set the tone for a wretched Hinrunde: only four wins in 17 games left die Schwarz-Gelben a place from bottom over the winter break. The unthinkable did happen when BVB slipped to 18th in February, although coach Jürgen Klopp masterminded a revival-of-sorts, inspiring his troops to a seventh-place finish and UEFA Europa League qualification. The legendary tactician called time on his era at the Signal Iduna Park at the end of the campaign, his final game a 3-1 defeat to VfL Wolfsburg in the DFB Cup final.

Ins...

Gonzalo Castro (r.) (Bayer 04 Leverkusen)

  • Versatile 28-year-old midfielder who joins from Leverkusen, where he had spent his whole career.
  • Recorded nine assists in 2014/15, while making 22 of his 286 Bundesliga appearances.

What he offers: Creativity, bags of domestic and continental experience and the versatility to play deeper or further forward in midfield.

Roman Bürki (SC Freiburg)

  • Talented Swiss shot-stopper, 24.
  • Made an astonishing 152 saves in 34 games last season, more than any other keeper in the league.

What he offers: Excellent reflexes, command of his area and dominant in one-on-one situations.

Julian Weigl (TSV 1860 Munich)

  • One of Germany's most talented central midfield prospects, 19.
  • Became 1860's youngest ever captain last season, making 24 appearances as his side avoided relegation from Bundesliga 2 in the final seconds of the season.

What he offers: Leadership, superb vision and strength in the tackle.

Outs...

Mitchell Langerak (r.) (VfB Stuttgart)

  • Australian international goalkeeper, 26.
  • Won two Bundesliga titles and one DFB Cup during his time at BVB.

Ciro Immobile (Sevilla FC)

  • Twelve-time Italian international centre-forward, 25.
  • Only managed three goals in 24 Bundesliga appearances with Dortmund.

Milos Jojic (1. FC Köln)

  • Talented 23-year-old Serbian central midfielder.
  • Found first-team opportunities limited in 2014/15, turning out on only ten occasions.

To-do list...

Charged with filling Klopp's shoes in the BVB hotseat, Thomas Tuchel (l.) will still have one or two areas of improvement in mind ahead of the visit of Borussia Mönchengladbach on Matchday 1. Right-back Lukasz Piszczek has been regularly struck down by injury in recent seasons, so either competition or back-up for the Pole would come in handy. The arrivals of Castro and Weigl reinforce a defensive midfield which looked a touch light last season, but with only Adrian Ramos and Pierre-Emerick Aubameyang as recognised strikers, signing a proven goalscorer is a pressing need for die Schwarz-Gelben.

Editor's note: Not all summer transfers will be discussed here. The players featured in this article made at least ten league appearances for their respective clubs last season, chipped in with numerous goals and assists or made other significant contributions to their team. For an overview of the 2015/16 summer transfers, click here.